Fix VS2015 sln icon

This is a minor fix to make the correct sln icon appear in Windows. The
Icon Handler for sln files is apparently quite picky.

VS2015 sln files contain this:
# Visual Studio 14

Not this:
# Visual Studio 2015

Having 2015 instead of 14 does not appear to cause any major issues. But
you get the generic sln file icon in Explorer instead of the
VS2015-specific one.

src/actions/vstudio/vs2015.lua

@@ -65,7 +65,7 @@
 
 		vstudio = {
 			solutionVersion = "12",
-			versionName     = "2015",
+			versionName     = "14",
 			targetFramework = "4.5",
 			toolsVersion    = "14.0",
 			filterToolsVersion = "4.0",