pry_class.rb: do not monkey-patch CodeRay anymore

The new CodeRay version (1.1.0) has changed its internal structure for
terminal encoder colours. We could update our patch and apply the new
syntax, but during the discussion with CodeRay's author we both agreed
that the vanilla colour looks much more pleasant. So I decided to remove
the patch altogether. The dark blue colour looks *really* awful on black
background.

See: https://github.com/rubychan/coderay/issues/154