drop 5 and 0.12 on appveyor, add 6

Author: isaacs

appveyor.yml

@@ -1,8 +1,7 @@
 environment:
   matrix:
-    - nodejs_version: '5'
+    - nodejs_version: '6'
     - nodejs_version: '4'
-    - nodejs_version: '0.12'
 install:
   - ps: Install-Product node $env:nodejs_version
   - set CI=true