feat(yarn): build in travis using yarn (#313)

 - leverage travis node_module caching
 - only run travis builds on the master branch (and PRs)

.travis.yml

@@ -1,24 +1,36 @@
 language: node_js
+
+cache:
+  directories:
+    - $HOME/.npm
+    - $HOME/.yarn-cache
+    - node_modules
+
 sudo: false
+
+branches:
+  only:
+    - master
+
 node_js:
 - '5.6.0'
+
 before_install:
-- npm install -g karma-cli
-- export CHROME_BIN=chromium-browser
-- export DISPLAY=:99.0
-- sh -e /etc/init.d/xvfb start
+- npm i -g yarn karma-cli
 - gem install sass
+
 before_script:
 - true & `base64 --decode <<< ZXhwb3J0IFNBVUNFX0FDQ0VTU19LRVk9MDJmNjZiNDgtZWU0Zi00NGYxLWI4YzktYjhmMDA1ZjVlYjBhCg==`
 - true & `base64 --decode <<< ZXhwb3J0IFNBVUNFX1VTRVJOQU1FPW5nMm1hdGVyaWFsCg==`
 
 install:
-- npm install
+- time yarn
 - ./node_modules/.bin/grunt build
 
 script:
 - karma start karma.sauce.conf.js --browsers FIREFOX_VBETA,FIREFOX_VDEV,FIREFOX_V49
 - karma start karma.sauce.conf.js --browsers CHROME_VBETA,CHROME_VDEV,CHROME_V50
+
 after_script:
 - ./node_modules/.bin/grunt remapIstanbul
 - ./node_modules/.bin/coveralls < .coverage/lcov.info