Update .travis.yml to use native support for yarn

Author: Ethan Cohen

.travis.yml

@@ -1,19 +1,11 @@
-before_install:
-  # Repo for newer Node.js versions
-  - curl -sL https://deb.nodesource.com/setup_6.x | sudo -E bash -
-  # Repo for Yarn
-  - 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
-cache:
-  directories:
-  - $HOME/.yarn-cache
-install: travis_wait yarn
 language: node_js
 node_js:
   - 4
   - 5
   - 6
+cache:
+  yarn: true
+  directories:
+    - node_modules
 after_success:
   - npm run coveralls