Replaced the worker image name, which doesn't work as generator name, with a manually created generator name.

appveyor.yml

@@ -28,10 +28,11 @@ build:
   project: Assimp.sln
 
 install:
-  - echo off
   - set CMAKE_DEFINES -DASSIMP_WERROR=ON
-  - if "%platform%"=="x86" set CMAKE_GENERATOR_NAME=%APPVEYOR_BUILD_WORKER_IMAGE%
-  - if "%platform%"=="x64" set CMAKE_GENERATOR_NAME=%APPVEYOR_BUILD_WORKER_IMAGE% Win64
+  - 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 "%platform%"=="x64" set CMAKE_GENERATOR_NAME=%CMAKE_GENERATOR_NAME% Win64
   - cmake %CMAKE_DEFINES% -G "%CMAKE_GENERATOR_NAME%"
 
 cache: