fix(ci): update node.js support to >= 6.4

There are testing failures in 4.x and 5.x from
syntax issues in angular. 6.x was chosen as a
base since ES6 is on the roadmap for bacstack and
ES6 only has a 57% complete implementination

Author: dopry

.travis.yml

@@ -1,9 +1,10 @@
 language: "node_js"
 node_js:
+  - "9"
+  - "8"
   - "7"
   - "6"
-  - "5"
-  - "4"
+
 
 before_install:
   - "npm install coveralls"

package.json

@@ -29,7 +29,7 @@
     "email": "fabio@fh1.ch"
   },
   "engines": {
-    "node": ">= 0.12.0"
+    "node": ">= 6.4.0"
   },
   "commitlint": {
     "extends": [