Add msvc-14.0 to appveyor.yml

Author: pdimov

appveyor.yml

@@ -15,7 +15,7 @@ branches:
 environment:
   matrix:
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-      TOOLSET: msvc-12.0
+      TOOLSET: msvc-12.0,msvc-14.0
       ADDRMD: 32,64
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       TOOLSET: msvc-14.1,clang-win