Remove node 6-9 from tagged appveyor builds

Author: Prior99

appveyor.yml

@@ -25,18 +25,6 @@
   environment:
     NODE_LIBPNG_SKIP_BINARY_DOWNLOAD_FOR_CI: true
     matrix:
-      - nodejs_version: 6
-        GYP_MSVS_VERSION: 2015
-        APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-      - nodejs_version: 7
-        GYP_MSVS_VERSION: 2015
-        APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-      - nodejs_version: 8
-        GYP_MSVS_VERSION: 2015
-        APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-      - nodejs_version: 9
-        GYP_MSVS_VERSION: 2015
-        APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
       - nodejs_version: 10
         GYP_MSVS_VERSION: 2015
         APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015