Merge pull request #253 from msgpack/fix/appveyor-xamarin

Fix appveyor settings tfor 0.9.0 release.

Author: yfakariya

appveyor-debug.yml

@@ -9,7 +9,7 @@ assembly_info:
   assembly_file_version: $(AssemblyBaseVersion).{build}
   assembly_informational_version: $(PackageVersion)
 environment:
-  XamarinMSBuildExtensionsPath: C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\
+  XamarinMSBuildExtensionsPath: C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\MSBuild
 install:
 - cmd: >-
     cd .\build

appveyor-release.yml

@@ -14,7 +14,7 @@ assembly_info:
   assembly_file_version: $(AssemblyBaseVersion).{build}
   assembly_informational_version: $(PackageVersion)
 environment:
-  XamarinMSBuildExtensionsPath: C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\
+  XamarinMSBuildExtensionsPath: C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\MSBuild
 install:
 - cmd: >-
     cd .\build