update Node to 6.x LTS (#1228)

The 4.x branch [entered maintenance](https://github.com/nodejs/LTS#lts-schedule1) on 1 April.

.nvmrc

@@ -1 +1 @@
-6.7.0
+6

.travis.yml

@@ -5,8 +5,6 @@ notifications:
   email: false
 
 env:
-  matrix:
-    - TRAVIS_NODE_VERSION="4"
   global:
     - LOCAL_DOMAIN=cb6e6126.ngrok.io
     - LOCAL_HTTPS=true
@@ -28,8 +26,7 @@ before_install:
   - sudo apt-get -qq update
   - sudo apt-get -qq install g++-4.8
 install:
-  - nvm install $TRAVIS_NODE_VERSION
-  - npm install -g npm@3
+  - nvm install
   - npm install -g yarn
   - bundle install
   - yarn install