Update Composer dependencies (2019-08-19)

Also sets PHP 7.0 as a platform requirement to resolve WP.org linting
issue, see #412

```
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 61 installs, 0 updates, 0 removals
  - Installing squizlabs/php_codesniffer (3.4.2): Loading from cache
  - Installing dealerdirect/phpcodesniffer-composer-installer (v0.5.0): Loading from cache
  - Installing ralouphie/getallheaders (3.0.3): Loading from cache
  - Installing psr/http-message (1.0.1): Loading from cache
  - Installing guzzlehttp/psr7 (1.6.1): Loading from cache
  - Installing guzzlehttp/promises (v1.3.1): Loading from cache
  - Installing guzzlehttp/guzzle (6.3.3): Loading from cache
  - Installing symfony/polyfill-mbstring (v1.12.0): Loading from cache
  - Installing symfony/polyfill-ctype (v1.12.0): Loading from cache
  - Installing symfony/dom-crawler (v3.3.6): Loading from cache
  - Installing symfony/css-selector (v3.3.6): Loading from cache
  - Installing symfony/browser-kit (v3.3.6): Loading from cache
  - Installing fabpot/goutte (v3.2.3): Loading from cache
  - Installing psr/container (1.0.0): Loading from cache
  - Installing behat/mink (v1.7.1): Loading from cache
  - Installing behat/mink-browserkit-driver (1.3.3): Loading from cache
  - Installing behat/mink-goutte-driver (v1.2.1): Loading from cache
  - Installing symfony/filesystem (v3.3.6): Loading from cache
  - Installing symfony/config (v3.3.6): Loading from cache
  - Installing container-interop/container-interop (1.2.0): Loading from cache
  - Installing symfony/class-loader (v3.3.6): Loading from cache
  - Installing symfony/yaml (v3.3.6): Loading from cache
  - Installing symfony/translation (v3.3.6): Loading from cache
  - Installing symfony/dependency-injection (v3.3.6): Loading from cache
  - Installing psr/log (1.1.0): Loading from cache
  - Installing symfony/debug (v3.0.9): Loading from cache
  - Installing symfony/console (v2.8.50): Loading from cache
  - Installing behat/transliterator (v1.2.0): Loading from cache
  - Installing behat/gherkin (v4.6.0): Loading from cache
  - Installing behat/behat (v3.5.0): Loading from cache
  - Installing behat/mink-extension (2.3.1): Loading from cache
  - Installing pantheon-systems/pantheon-wordpress-upstream-tests (dev-master 2f25dde): Cloning 2f25ddefdd from cache
  - Installing wp-coding-standards/wpcs (2.1.1): Loading from cache
  - Installing sebastian/version (2.0.1): Loading from cache
  - Installing sebastian/resource-operations (1.0.0): Loading from cache
  - Installing sebastian/object-reflector (1.1.1): Loading from cache
  - Installing sebastian/recursion-context (3.0.0): Loading from cache
  - Installing sebastian/object-enumerator (3.0.3): Loading from cache
  - Installing sebastian/global-state (2.0.0): Loading from cache
  - Installing sebastian/exporter (3.1.1): Loading from cache
  - Installing sebastian/environment (3.1.0): Loading from cache
  - Installing sebastian/diff (2.0.1): Loading from cache
  - Installing sebastian/comparator (2.1.3): Loading from cache
  - Installing doctrine/instantiator (1.0.5): Loading from cache
  - Installing phpunit/php-text-template (1.2.1): Loading from cache
  - Installing phpunit/phpunit-mock-objects (5.0.10): Loading from cache
  - Installing phpunit/php-timer (1.0.9): Loading from cache
  - Installing phpunit/php-file-iterator (1.4.5): Loading from cache
  - Installing theseer/tokenizer (1.1.3): Loading from cache
  - Installing sebastian/code-unit-reverse-lookup (1.0.1): Loading from cache
  - Installing phpunit/php-token-stream (2.0.2): Loading from cache
  - Installing phpunit/php-code-coverage (5.3.2): Loading from cache
  - Installing webmozart/assert (1.4.0): Loading from cache
  - Installing phpdocumentor/reflection-common (1.0.1): Loading from cache
  - Installing phpdocumentor/type-resolver (0.4.0): Loading from cache
  - Installing phpdocumentor/reflection-docblock (4.3.1): Loading from cache
  - Installing phpspec/prophecy (1.8.1): Loading from cache
  - Installing phar-io/version (1.0.1): Loading from cache
  - Installing phar-io/manifest (1.0.1): Loading from cache
  - Installing myclabs/deep-copy (1.7.0): Loading from cache
  - Installing phpunit/phpunit (6.5.14): Loading from cache
guzzlehttp/psr7 suggests installing zendframework/zend-httphandlerrunner (Emit PSR-7 responses)
symfony/browser-kit suggests installing symfony/process
behat/mink suggests installing behat/mink-selenium2-driver (slow, but JS-enabled driver for any app (requires Selenium2))
behat/mink suggests installing behat/mink-zombie-driver (fast and JS-enabled headless driver for any app (requires node.js))
symfony/class-loader suggests installing symfony/polyfill-apcu (For using ApcClassLoader on HHVM)
symfony/dependency-injection suggests installing symfony/finder (For using double-star glob patterns or when GLOB_BRACE portability is required)
symfony/dependency-injection suggests installing symfony/expression-language (For using expressions in service container configuration)
symfony/dependency-injection suggests installing symfony/proxy-manager-bridge (Generate service proxies to lazy load them)
symfony/console suggests installing symfony/process
symfony/console suggests installing psr/log-implementation (For using the console logger)
sebastian/global-state suggests installing ext-uopz (*)
phpunit/php-code-coverage suggests installing ext-xdebug (^2.5.5)
phpunit/phpunit suggests installing phpunit/php-invoker (^1.1)
phpunit/phpunit suggests installing ext-xdebug (*)
Package phpunit/phpunit-mock-objects is abandoned, you should avoid using it. No replacement was suggested.
Writing lock file
Generating autoload files
PHP CodeSniffer Config installed_paths set to ../../wp-coding-standards/wpcs
```

composer.json

@@ -14,14 +14,19 @@
     },
     "minimum-stability": "dev",
     "prefer-stable": true,
+    "config": {
+        "platform": {
+            "php": "7.0"
+        }
+    },
     "require": {
         "solarium/solarium": "~4"
     },
     "require-dev": {
         "pantheon-systems/pantheon-wordpress-upstream-tests": "dev-master",
         "wp-coding-standards/wpcs": "^2.1",
         "dealerdirect/phpcodesniffer-composer-installer": "^0.5.0",
-        "phpunit/phpunit": "^7"
+        "phpunit/phpunit": "^6 || ^7"
     },
     "scripts": {
         "lint": "@phpcs",