Add enh-ruby-mode specific faces

* Define faces for enh-ruby-mode

`enh-ruby-mode` has some specific faces for string, heredoc, regexp delimiters etc. It is setting up them in the hook [here](https://github.com/zenspider/enhanced-ruby-mode/blob/master/enh-ruby-mode.el#L198). So in order to not mix definitions of faces from `solarized-emacs` and `enh-ruby-mode` this should be added to the user's config: `(remove-hook 'enh-ruby-mode-hook 'erm-define-faces)`. 

I'm not sure how to approach this. `solarized-emacs` should call this `remove-hook`? Or it should be done by the user? If yes, should it be mentioned in the doc? Or maybe `enh-ruby-mode` should not add this hook by default. What do you think @zenspider ?