chore: remove support for node 6 (#3246)

Set node 8 as minimal supported version.

Closes #3151
BREAKING CHANGE:
Drop Support for Node 6, to make it possible to use async/await in karma codebase.

.travis.yml

@@ -1,7 +1,6 @@
 sudo: false
 language: node_js
 node_js:
-  - 6
   - 8
   - 10
 

appveyor.yml

@@ -2,7 +2,6 @@ version: "{build}"
 
 environment:
   matrix:
-    - nodejs_version: "6"
     - nodejs_version: "8"
     - nodejs_version: "10"
 

docs/intro/04-faq.md

@@ -25,8 +25,7 @@ The latest stable version from NPM (`npm install karma`). See [versioning] for m
 
 
 ### Which version of Node.js does Karma run with?
-Karma works on all LTS versions node in active maintenance state (see [LTS docs](https://github.com/nodejs/Release/blob/master/README.md) for more info) as well as the latest stable version. That is **6.x**, **8.x** and **10.x** at this point.
-
+Karma works on all active LTS versions of node as specified by the [Node.js Release Working Group](https://github.com/nodejs/Release/blob/master/README.md). The nodejs version numbers are set in the package.json. Older versions of karma work with older versions of nodejs, but are not maintained or updated.
 
 [mailing list]: https://groups.google.com/d/forum/karma-users
 [karma-ng-scenario]: https://github.com/karma-runner/karma-ng-scenario

package.json

@@ -475,7 +475,7 @@
     "karma": "./bin/karma"
   },
   "engines": {
-    "node": ">= 6"
+    "node": ">= 8"
   },
   "version": "3.1.4",
   "license": "MIT",