Revert "[ci] - Test yarn in travis ci."

This reverts commit b60a3f0.

Author: Ethan Cohen

.travis.yml

@@ -3,14 +3,5 @@ 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