chore(node): allow current node version

node current is 6.6.0
see https://github.com/nodejs/LTS#lts_schedule

.nvmrc

@@ -1 +1 @@
-5.4.1
+6.6.0

.travis.yml

@@ -1,7 +1,7 @@
 language: node_js
 sudo: false
 node_js:
-  - '5.4.1'
+  - '6.6.0'
 
 addons:
 #  firefox: "38.0"

gulpfile.js

@@ -4,7 +4,7 @@
 // This is to ensure that we catch env issues before we error while requiring other dependencies.
 require('./tools/check-environment')({
   requiredNpmVersion: '>=3.5.3 <4.0.0',
-  requiredNodeVersion: '>=5.4.1 <6.0.0',
+  requiredNodeVersion: '>=5.4.1 <7.0.0',
 });
 
 const gulp = require('gulp');

package.json

@@ -8,7 +8,7 @@
   "bugs": "https://github.com/angular/angular/issues",
   "license": "MIT",
   "engines": {
-    "node": ">= 5.4.1 < 6"
+    "node": ">= 5.4.1 < 7"
   },
   "repository": {
     "type": "git",