Try fixing yarn to 0.16.1

Ref yarnpkg/yarn#1838

.travis.yml

@@ -17,7 +17,9 @@ before_install:
   - 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
+  # trying 0.16.1 because https://github.com/yarnpkg/yarn/issues/1838
+  - sudo apt-cache policy yarn
+  - sudo apt-get install -y -qq yarn=0.16.1-1
 
 # 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()