Drop build/test support for Node.js 6

Some npm packages, including some of our build/test dependencies have
begun dropping support for Node.js 6 in their latest versions. To
facilitate dependency updates, we remove support for Node.js 6 here.

This package is usually not built and tested on our user's machines,
but is instead bundled with the platforms by us. Thus, this change
should not be considered a breaking change.

.travis.yml

@@ -4,7 +4,6 @@ git:
   depth: 10
 
 node_js:
-  - "6"
   - "8"
   - "10"
 

appveyor.yml

@@ -2,7 +2,6 @@
 # http://www.appveyor.com/docs/appveyor-yml
 environment:
   matrix:
-  - nodejs_version: "6"
   - nodejs_version: "8"
   - nodejs_version: "10"
 

package.json

@@ -3,7 +3,7 @@
   "name": "cordova-js",
   "description": "Cordova JavaScript: a unified JavaScript layer for the Cordova suite of projects enabling cross-platform native mobile development of applications using HTML, CSS and JavaScript.",
   "engines": {
-    "node": ">=6.0.0"
+    "node": ">=8.0.0"
   },
   "version": "5.0.1-dev",
   "homepage": "http://cordova.apache.org",