fix(travis): manually install yarn on Travis CI

Author: armandabric

.travis.yml

@@ -5,3 +5,9 @@ branches:
     - master
     - /^greenkeeper\/.*$/
 cache: yarn
+
+# Since the upgrade to Ubuntu Precise, yarn is not available to run scripts: `yarn run foo`
+# So let's install it manually
+before_install:
+  - curl -o- -L https://yarnpkg.com/install.sh | bash
+  - export PATH="$HOME/.yarn/bin:$PATH"