Add nvm install and nvm use to travis.yml

Author: dylangrafmyre

.travis.yml

@@ -2,8 +2,9 @@ language:
   - ruby
 rvm:
   - 2.2.2
-nvm:
-  - 4.2.0
+before_install:
+  - nvm install 4.2.0
+  - nvm use 4.2.0
 install:
   - bundle install
   - npm install -g npm