Upgrade AppVeyor to VS 2015

Daniel Kroening · Daniel Kroening · commit b8185ac69b33 · 2018-07-13T07:36:42.000+01:00
diff --git a/appveyor.yml b/appveyor.yml
@@ -1,5 +1,5 @@
 version: 1.0.{build}
-image: Visual Studio 2013
+image: Visual Studio 2015
 clone_depth: 50
 environment:
   PATH: C:\projects\cbmc\deps\bin;%PATH%
