chore: remove node 6 testing from CI (Appveyor, Travis)

Karma 4.0 drops node 6 support,
and the node 6 builds have caused many unnecessary build failures in CI.

.appveyor.yml

@@ -2,7 +2,7 @@ image: Visual Studio 2017
 environment:
   timeout: 4000
   matrix:
-    - nodejs_version: "6"
+    # - nodejs_version: "6"
     # - nodejs_version: "7"
     - nodejs_version: "8" 
     # - nodejs_version: "10"

.travis.yml

@@ -2,7 +2,7 @@ sudo: required
 dist: trusty
 language: node_js
 node_js:
-- '6'
+# - '6'
 - '8'
 env:
   - timeout=10000