build(windows): remove nodejs v6

Author: gregswindle

appveyor.yml

@@ -7,14 +7,13 @@ environment:
     - nodejs_version: '11'
     - nodejs_version: '10'
     - nodejs_version: '8'
-    - nodejs_version: '6'
 install:
   - ps: Install-Product node $env:nodejs_version
   - npm i npm@latest -g
   - npm install
 matrix:
   fast_finish: true
-build: off
+build: 'off'
 shallow_clone: true
 clone_depth: 1
 test_script: