Update travis to use yarn

Author: joni2back

.travis.yml

@@ -1,10 +1,14 @@
 language: node_js 
 
+node_js:
+  - 6
+  
 sudo: false
 
-before_install:
-  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.3.2
-  - export PATH=$HOME/.yarn/bin:$PATH
+before_script:
+   - nvm install 6
+   - npm install -g yarn
+   - yarn install
 
 cache:
   yarn: true