Add yarn.lock and use yarn on CI

Author: lukemelia

.travis.yml

@@ -6,18 +6,13 @@ node_js:
 sudo: false
 
 cache:
+  yarn: true
   directories:
-    - $HOME/.npm
     - $HOME/.cache # includes bowers cache
 
-before_install:
-  - npm config set spin false
-  - npm install -g bower
-  - bower --version
-
-install:
-  - npm install
-  - bower install
+before_script:
+  - yarn global add bower # this would be better to do in install step,
+  - bower install         #   once travis' yarn support improves
 
 script:
-  - npm test
+  - yarn test