Specify the msvc 2015 image

Author: pfultz2

appveyor.yml

@@ -1,4 +1,6 @@
 
+os: Visual Studio 2015 RC
+
 environment:
   matrix:
   - GENERATOR: "Visual Studio 14 2015 Win64"
@@ -13,7 +15,7 @@ matrix:
 build_script:
  - cmd: set PATH=C:\Program Files (x86)\CMake\bin;%PATH%
  - cmd: set PATH=C:\Program Files (x86)\MSBuild\14.0\Bin;%PATH%
- - cmd: call "C:/Program Files (x86)/Microsoft Visual Studio 14.0/Common7/Tools/vsvars32.bat"
+ - cmd: call "C:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\Tools\vsvars32.bat"
  - cmd: cmake --version
  - cmd: mkdir build
  - cmd: cd build