chore(CI): use only node version 10 for CI builds instead of 6+8 or 8

node 10 is latest LTS version and much faster than 6
no node version problems beyond version 6 anyways
speeds up CI builds (Travis, AppVeyor)
discussed with @bneumann

.appveyor.yml

@@ -4,8 +4,8 @@ environment:
   matrix:
     # - nodejs_version: "6"
     # - nodejs_version: "7"
-    - nodejs_version: "8" 
-    # - nodejs_version: "10"
+    # - nodejs_version: "8" 
+    - nodejs_version: "10"
 platform:
   # - x86
   - x64

.travis.yml

@@ -3,7 +3,8 @@ dist: trusty
 language: node_js
 node_js:
 # - '6'
-- '8'
+# - '8'
+- '10'
 env:
   - timeout=10000
 notifications: