In Ruby `_$=@!:?` can all be part of an identifier. If we don't have
them in this list, autocomplete doesn't work as expected.
See: https://gist.github.com/misfo/1072693
This fixes one part of #7819 but not the whole ticket.
Release Notes:
- Fixed completions in Ruby not working for identifiers that start or
end with special characters (e.g.: `@`)
Thorsten Ball
3921259b6c