Call msbuild correctly, doh

Author: shana

appveyor.yml

@@ -44,7 +44,7 @@ install:
     nuget restore GitHub.Unity.sln
 
 before_build:
-  - cmd: msbuild GitHub.Unity.sln /target:OctoRun /Configuration:Release
+  - cmd: msbuild GitHub.Unity.sln /target:OctoRun /property:Configuration=Release
 
 assembly_info:
   patch: false