Enable built-in Ruby Mode, by default on Emacs 24.4 and newer where it
was significantly improved. Otherwise keep using Enh Ruby Mode.
Add all keybindings to both modes, and provide a variable to change the
default mode.
Helm seems to treat "!" specially in pattern matching, so having a ! in
the pattern string when traversing directories is problematic. This
change fixes#2737, because as far as I can tell "+" has no special
meaning in a helm pattern.
Of course, we can choose a different character, but I'm fond of "+" as
representing "more layers here".
