Support VS2017 on AppVeyor (#376)

appveyor.yml

 version: '{build}'
 
+image: Visual Studio 2017
+
 configuration:
   - Debug
   - Release
 
 environment:
   matrix:
-    - compiler: msvc-12-seh
-      generator: "Visual Studio 12 2013"
+    - compiler: msvc-15-seh
+      generator: "Visual Studio 15 2017"
 
-    - compiler: msvc-12-seh
-      generator: "Visual Studio 12 2013 Win64"
+    - compiler: msvc-15-seh
+      generator: "Visual Studio 15 2017 Win64"
 
     - compiler: msvc-14-seh
       generator: "Visual Studio 14 2015"
@@ -18,9 +20,16 @@ environment:
     - compiler: msvc-14-seh
       generator: "Visual Studio 14 2015 Win64"
 
+    - compiler: msvc-12-seh
+      generator: "Visual Studio 12 2013"
+
+    - compiler: msvc-12-seh
+      generator: "Visual Studio 12 2013 Win64"
+
     - compiler: gcc-5.3.0-posix
       generator: "MinGW Makefiles"
       cxx_path: 'C:\mingw-w64\i686-5.3.0-posix-dwarf-rt_v4-rev0\mingw32\bin'
+      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
 
 matrix:
   fast_finish: true
@@ -30,12 +39,6 @@ install:
   - if "%generator%"=="MinGW Makefiles" (set "PATH=%PATH:C:\Program Files\Git\usr\bin;=%")
   - if not "%cxx_path%"=="" (set "PATH=%PATH%;%cxx_path%")
 
-# TODO Remove this. This is a hack to work around bogus warning messages
-# See http://goo.gl/euguBI for more information.
-before_build:
-  - del "C:\Program Files (x86)\MSBuild\14.0\Microsoft.Common.targets\ImportAfter\Xamarin.Common.targets"
-  - del "C:\Program Files (x86)\MSBuild\12.0\Microsoft.Common.targets\ImportAfter\Xamarin.Common.targets"
-
 build_script:
   - md _build -Force
   - cd _build
@@ -51,4 +54,3 @@ artifacts:
     name: logs
   - path: '_build/Testing/**/*.xml'
     name: test_results
-