Update travis Node version

add `.nvrmc` for two reasons:

- travis uses `nvm` by default to manage node versions.
- It will make it easy for contributers who use `nvm` to use the proper
node version locally too.

Author: ahmedelgabri

.nvmrc

@@ -0,0 +1 @@
+6.3.0

.travis.yml

@@ -1,8 +1,6 @@
 sudo: false
 language: node_js
 env: TEST_SUITE=unit
-node_js:
-  - "5.6.0"
 cache:
   directories:
     - node_modules