Add Visual Studio 2015 image To .appveyor.yml

It is the image that turns on the magic, not the use of the build job.
I suspect this alone will not be enough to get it working and I will
need to use the ./clean_build.bat but we shall see.

.appveyor.yml

@@ -10,6 +10,7 @@ version: '0.0.{build}'
 #  common_var2: value2
 
 image:
+  - Visual Studio 2015
   - Ubuntu
 
 platform:
@@ -41,13 +42,10 @@ install:
 
 ##### Build Phase #####
 
-# Use build_script for our Linux builds and build for .NET
+# Use build_script for our own build script
 build_script:
   - sh: ./clean_build.sh
 
-build:
-  - cmd: ./clean_build.bat
-
 ##### Test Phase #####
 
 # Use test_script for our Linux tests and test for .NET