use 2 images to build on appveyor

appveyor.yml

-os: Visual Studio 2017
+image:
+- Visual Studio 2015
+- Visual Studio 2017
 
 version: "{branch}-{build}"
 
@@ -9,10 +11,10 @@ install:
   - ps: Import-Module CoApp
 
 build_script:
-  - ps: .\scripts\nuget_build.ps1
+  - ps: if ($env:APPVEYOR_BUILD_WORKER_IMAGE -eq "Visual Studio 2017"){.\scripts\nuget_build.ps1}
 
 test_script:
-  - ps: .\tests\autotest-appveyor.ps1
+  - ps: if ($env:APPVEYOR_BUILD_WORKER_IMAGE -eq "Visual Studio 2015"){.\tests\autotest-appveyor.ps1}
 
 artifacts:
   - path: .\scripts\*.nupkg
\ No newline at end of file