Remove symfony/symfony dependency

This allows us to be compatible with flex rollouts and also reduces the
footprint of the package in general. Testing now only needs to cover
higher/lower versions of these packages.

.travis.yml

@@ -3,23 +3,15 @@ language: php
 php:
     - 7.1.13
     - 7.2
+env:
+    matrix:
+    - PREFER_LOWEST="--prefer-lowest"
+    - PREFER_LOWEST=""
 
 before_script:
   - phpenv config-rm xdebug.ini
   - composer self-update
   - composer install --dev
+  - composer update $PREFER_LOWEST
 
 script: ./vendor/bin/phpunit
-
-matrix:
-  include:
-    - php: 7.2
-      env: SYMFONY_VERSION=3.0.*
-    - php: 7.2
-      env: SYMFONY_VERSION=^3
-    - php: 7.2
-      env: SYMFONY_VERSION=^4
-    - php: 7.2
-      env: SYMFONY_VERSION=dev-master@dev
-  allow_failures:
-    - env: SYMFONY_VERSION=dev-master@dev

composer.json

@@ -16,7 +16,9 @@
     "require": {
         "php":  "^7.1.13",
         "dms/dms-filter": "^2.0 | ^3.0",
-        "symfony/symfony": "^2.8.18 | ^3.0 | ^4.0"
+        "symfony/yaml": "^3.4 | ^4.0",
+        "symfony/framework-bundle": "^3.4 | ^4.0",
+        "symfony/form": "^3.4 | ^4.0"
     },
 
     "require-dev": {