Add Visual Studio 2015 to the tested toolchains in Appveyor

* Closes #2844

appveyor.yml

@@ -5,8 +5,12 @@
 
 version: '{branch}-{build}'
 
-image:
-  - Visual Studio 2017
+environment:
+  matrix:
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+      MSVC_GENERATOR: Visual Studio 14 2015
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+      MSVC_GENERATOR: Visual Studio 15 2017
 
 configuration:
   - Debug
@@ -25,7 +29,7 @@ install:
   - cmd: 7z x "C:\projects\vcpkg-export-hpx-dependencies.7z" -y -oC:\projects\vcpkg >NUL
 
 before_build:
-  - cmd: cmake -H. -Bbuild -A%PLATFORM% -DCMAKE_BUILD_TYPE=%CONFIGURATION% -DCMAKE_TOOLCHAIN_FILE=C:/projects/vcpkg/scripts/buildsystems/vcpkg.cmake -DHPX_WITH_GIT_COMMIT=%APPVEYOR_REPO_COMMIT% -DHPX_WITH_PSEUDO_DEPENDENCIES=On -DHPX_WITH_EXAMPLES=On -DHPX_WITH_EXAMPLES_HDF5=Off -DHPX_WITH_TESTS=Off -DHPX_WITH_RUNTIME=Off -DHPX_WITH_DEPRECATION_WARNINGS=Off
+  - cmd: cmake -H. -Bbuild -G"%MSVC_GENERATOR%" -A%PLATFORM% -DCMAKE_BUILD_TYPE=%CONFIGURATION% -DCMAKE_TOOLCHAIN_FILE=C:/projects/vcpkg/scripts/buildsystems/vcpkg.cmake -DHPX_WITH_GIT_COMMIT=%APPVEYOR_REPO_COMMIT% -DHPX_WITH_PSEUDO_DEPENDENCIES=On -DHPX_WITH_EXAMPLES=On -DHPX_WITH_EXAMPLES_HDF5=Off -DHPX_WITH_TESTS=Off -DHPX_WITH_RUNTIME=Off -DHPX_WITH_DEPRECATION_WARNINGS=Off
 
 build_script:
   - cmd: cmake --build build --config %CONFIGURATION% --target core -- /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll" /verbosity:minimal /maxcpucount:2 /nologo