[ci] - Test yarn in travis ci.

Author: Ethan Cohen

.travis.yml

@@ -3,5 +3,14 @@ node_js:
   - 4
   - 5
   - 6
+cache:
+  directories:
+    - $HOME/.npm
+    - $HOME/.yarn-cache
+    - node_modules
+before_install:
+  - time npm i -g yarn --cache-min 999999999
+install:
+  - time yarn
 after_success:
   - npm run coveralls