Try fixing yarn to 0.16.1

Ref yarnpkg/yarn#1838

.travis.yml

@@ -10,14 +10,7 @@ matrix:
   fast_finish: true
 
 before_install:
-  # https://yarnpkg.com/en/docs/install-ci#travis-tab
-  # Repo for newer Node.js versions
-  - curl -sL https://deb.nodesource.com/setup_6.x | sudo -E bash -
-  # Repo for Yarn
-  - sudo apt-key adv --fetch-keys http://dl.yarnpkg.com/debian/pubkey.gpg
-  - echo "deb http://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list
-  - sudo apt-get update -qq
-  - sudo apt-get install -y -qq yarn
+  - npm install -g [email protected]
 
 # cache node modules
 cache:

appveyor.yml

@@ -4,6 +4,9 @@ environment:
   matrix:
     - nodejs_version: '6'
 
+  # stick to npm
+  PHENOMIC_TEST_WITH_YARN: 0
+
 version: "{build}"
 build: off
 deploy: off

scripts/utils/test-folder.js

@@ -27,13 +27,15 @@ export default async function test(
   try {
     const targetModules = `${ target }/node_modules`
 
-    let yarn
-    try {
-      await exec("yarn --version")
-      yarn = true
-    }
-    catch (e) {
-      yarn = false
+    let yarn = process.env.PHENOMIC_TEST_WITH_YARN || true
+    if (yarn) {
+      try {
+        await exec("yarn --version")
+        yarn = true
+      }
+      catch (e) {
+        yarn = false
+      }
     }
 
     await cleanup()