Use Yarn instead (#904)

* Install yarn as part of the install phase
* Cache the yarn directory
* Cache .nvm

Author: MrRio

.travis.yml

@@ -1,3 +1,10 @@
 language: node_js
 node_js:
   - "4"
+install:
+  - npm -g install yarn
+  - yarn
+cache:
+  directories:
+    - ~/.yarn
+    - ~/.nvm