AppVeyor: activate VS2015

appveyor.yml

@@ -12,6 +12,7 @@ configuration:
 
 environment:
   matrix:
+    - VSVER: 14
     - VSVER: 12
     - VSVER: 10
 
@@ -20,6 +21,12 @@ matrix:
     # not included in AppVeyor right now
     - platform: x64
       VSVER: 10
+    # does currently not build zlib out of the box
+    - configuration: Release
+      VSVER: 14
+    # does currently not build zlib out of the box
+    - configuration: Debug
+      VSVER: 14
 
 install:
   - ps: if ($env:APPVEYOR_PULL_REQUEST_NUMBER -and $env:APPVEYOR_BUILD_NUMBER -ne ((Invoke-RestMethod `