appveyor: Upgrade to Visual Studio 2015.

Special care is neeed to use Windows 10 SDK.

appveyor.yml

@@ -14,17 +14,17 @@ shallow_clone: true
 environment:
   MSBUILD_FLAGS: /verbosity:minimal /maxcpucount
   matrix:
-  - CMAKE_GENERATOR: "Visual Studio 12"
-    QT5: C:\Qt\5.6\msvc2013
-  - CMAKE_GENERATOR: "Visual Studio 12 Win64"
-    QT5: C:\Qt\5.6\msvc2013_64
+  - CMAKE_GENERATOR: "Visual Studio 14 2015"
+    QT5: C:\Qt\5.6\msvc2015
+  - CMAKE_GENERATOR: "Visual Studio 14 2015 Win64"
+    QT5: C:\Qt\5.6\msvc2015_64
 
 matrix:
   fast_finish: true
 
 before_build:
 - set Path=%QT5%\bin;%Path%
-- cmake -H. -Bbuild -G "%CMAKE_GENERATOR%" "-DCMAKE_PREFIX_PATH=%QT5%" -DENABLE_GUI=ON
+- cmake -H. -Bbuild -G "%CMAKE_GENERATOR%" -DCMAKE_SYSTEM_VERSION=10.0 "-DCMAKE_PREFIX_PATH=%QT5%" -DENABLE_GUI=ON
 
 build_script:
 - if "%APPVEYOR_REPO_TAG%"=="true" (set CONFIGURATION=RelWithDebInfo) else (set CONFIGURATION=Debug)