Vim config updates

* Ditch rainbow_parentheses for rainbow.vim
* Configure snippets using Joe Ferris' Ruby-aware vimscript along with
  ultisnips
* Settle on Ultisnips as snippet tool
* Settle on YouCompleteMe as autocomplete tool
* Default to generic omnicomplete for semantic completion in vim until
  neovim's Ruby support improves
* Don't default to Perl/Ruby-style regex, but retain the toggle through
  eregex.vim