Workaround appveyor issue where it doesn't import nuget.props

AArnott · AArnott · commit 2ed5fd9239d6 · 2017-02-18T11:33:09.000-08:00
See appveyor/ci#1179 (comment)
diff --git a/appveyor.yml b/appveyor.yml
@@ -10,6 +10,7 @@ init:
 
     git config --global user.email me@appveyor.com
 environment:
+  VisualStudioVersion: 15.0
   NuGetApiKey:
     secure: Ql19CstT9GvFNeSIAjXvgHXZg+/pNaYJ2jVkzde4LT2vwSyjiQgQ+M332wVYtu+r
 install:
