add *.vs/ to ignore VS files in VS15 CTP5

As of Visual Studio 2015 CTP5 the Roslyn cache and suo files are stored in a .vs/ directory.

VisualStudio.gitignore

@@ -25,6 +25,9 @@ bld/
 # Roslyn cache directories
 *.ide/
 
+# Visual Studo 2015 cache/options directory
+.vs/
+
 # MSTest test Results
 [Tt]est[Rr]esult*/
 [Bb]uild[Ll]og.*