[ci] - Use yarn in travis build. (#121)

Author: beefancohen

.travis.yml

@@ -1,3 +1,15 @@
+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