message: Add VS17 image to appveyor matrix
diff: version: '{build}'
  -os: Visual Studio 2015
  +os:
  +- Visual Studio 2015
  +- Visual Studio 2017
  init: []
  install: []
  build_script:
  -- set PATH=C:\Program Files (x86)\MSBuild\14.0\Bin;%PATH%
  -- cmake . -G "Visual Studio 14 2015"
  +- IF "%APPVEYOR_BUILD_WORKER_IMAGE%" == "Visual Studio 2015" ( SET GEN="Visual Studio 14 2015") ELSE (SET GEN="Visual Studio 15 2017")
  +- cmake . -G%GEN%
  - cmake --build . --config Release
  test_script:
  - ctest -C Release -V
  