message: use 2 images to build on appveyor
diff: -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
  