Update appveyor.yml

Fix cmake generation.

appveyor.yml

@@ -28,8 +28,8 @@ install:
   - set PATH=C:\Ruby24-x64\bin;%PATH%
   - set CMAKE_DEFINES -DASSIMP_WERROR=ON
   - if "%APPVEYOR_BUILD_WORKER_IMAGE%"=="Visual Studio 2013" set CMAKE_GENERATOR_NAME=Visual Studio 12 2013
-  - if "%APPVEYOR_BUILD_WORKER_IMAGE%"=="Visual Studio 2015" set CMAKE_GENERATOR_NAME=Visual Studio 14 2015
-  - if "%APPVEYOR_BUILD_WORKER_IMAGE%"=="Visual Studio 2017" set CMAKE_GENERATOR_NAME=Visual Studio 15 2017
+  - if "%APPVEYOR_BUILD_WORKER_IMAGE%"=="Previous Visual Studio 2015" set CMAKE_GENERATOR_NAME=Visual Studio 14 2015
+  - if "%APPVEYOR_BUILD_WORKER_IMAGE%"=="Previous Visual Studio 2017" set CMAKE_GENERATOR_NAME=Visual Studio 15 2017
   - if "%platform%"=="x64" set CMAKE_GENERATOR_NAME=%CMAKE_GENERATOR_NAME% Win64
   - cmake %CMAKE_DEFINES% -G "%CMAKE_GENERATOR_NAME%"
   - set PATH=%PATH%;"C:\\Program Files (x86)\\Inno Setup 5"