Unslashify *.gem,*.rbc,.rvmrc .

arcresu pointed out that `*.gem`, `*.rbc`, `.rvmrc` might not be at
the top level in a repository. Thus, remove leading slashes for these.

Ruby.gitignore

-/*.gem
-/*.rbc
+*.gem
+*.rbc
 /.config
 /coverage/
 /InstalledFiles
@@ -26,4 +26,4 @@
 # .ruby-gemset
 
 # unless supporting rvm < 1.11.0 or doing something fancy, ignore this:
-/.rvmrc
+.rvmrc