Update AppVeyor

appveyor.yml

@@ -1,6 +1,7 @@
 version: 1.2.{build}
+os: Visual Studio 2015
 before_build:
-- nuget restore src/Abc.Zebus.sln
+- cmd: nuget restore src/Abc.Zebus.sln
 build:
   verbosity: minimal
 test:
@@ -9,4 +10,4 @@ test:
     except:
     - ManualOnly
 cache:
-  - src\packages -> **\packages.config 
+  - src\packages -> **\packages.config