Update yarn version

Author: mmermerkaya

.travis.yml

@@ -10,7 +10,7 @@ env:
 before_install:
   # Yarn defaults to an old version, make sure we
   # get an up to date version
-  - npm install -g yarn@latest
+  - npm install -g yarn@1.15.2
   - '[ "${NPM_TOKEN+x}" ] && echo "//registry.npmjs.org/:_authToken=\${NPM_TOKEN}" > $HOME/.npmrc || echo "Skipping .npmrc creation";'
 install:
   - yarn install --frozen-lockfile