Update AppVeyor image to pick up newer NuGet

bording · bording · commit 3c8345162782 · 2017-04-01T17:18:43.000-04:00
diff --git a/appveyor.yml b/appveyor.yml
@@ -7,6 +7,8 @@ branches:
 
 skip_tags: true
 
+image: Visual Studio 2015
+
 clone_folder: C:\projects\libgit2sharp
 
 environment:
