Added VS 2015 test to AppVeyor CI.

Change-Id: I354c13fbe6f159058fbeff47100ef357e99c6cd5

appveyor.yml

@@ -4,6 +4,11 @@ branches:
 
 os: Visual Studio 2015
 
+environment:
+  matrix:
+    - CMAKE_VS_VERSION: "10 2010"
+    - CMAKE_VS_VERSION: "14 2015"
+
 platform:
   - x86
   - x64
@@ -13,7 +18,7 @@ configuration:
   - Release
 
 before_build:
-  - cmake -G"Visual Studio 10 2010"
+  - cmake -G"Visual Studio %CMAKE_VS_VERSION%"
   # This cuts down on a lot of noise generated by xamarin warnings.
   - del "C:\Program Files (x86)\MSBuild\14.0\Microsoft.Common.targets\ImportAfter\Xamarin.Common.targets"