Build: Remove Node 4 from AppVeyor test matrix

The AppVeyor build is currently failing due to Node 4 not
being supported anymore, whih is intentional and removed from
the Travi CI config already, but in reviewing #1288, I forgot
about AppVeyor :)

Closes #1346.

Author: Krinkle

appveyor.yml

@@ -3,7 +3,6 @@
 # Test against this version of Node.js
 environment:
   matrix:
-    - nodejs_version: "4"
     - nodejs_version: "6"
     - nodejs_version: "7"
     - nodejs_version: "8"