Make $, : and @ "prefix characters" in ruby-mode

Make $, : and @ "prefix characters" in ruby-mode

* lisp/progmodes/ruby-mode.el (ruby-mode-syntax-table): Change the
syntax classes of $, : and @ to "prefix character"
(http://lists.gnu.org/archive/html/emacs-devel/2016-01/msg00272.html).
(ruby-syntax-propertize): Undo that specifically for colons
followed by an opening paren or bracket.
(ruby-font-lock-keyword-beg-re): Include colon character.
(ruby-font-lock-keywords): Adjust the constants matcher for `:'
not being a symbol constituent anymore.