speed vim up when editing ruby files by changing regex engine

Vim sometimes get very slow when we try to edit a big (~500 lines) ruby file. This is due to the regex matching of the syntax highlighting.
Solutions:
1. We can speed it up by using an old version of regex engine. 
2. We can further speed it up by disable some rarely used syntax highlighting by comment the following lines in vim//bundle/vim-ruby/syntax/ruby.vim:
```vim
" syn match  rubySymbol   "\%([{(,]\_s*\)\@<=\l\w*[!?]\=::\@!"he=e-1
" syn match  rubySymbol   "[]})\"':]\@<!\%(\h\|[^\x00-\x7F]\)\%(\w\|[^\x00-\x7F]\)*[!?]\=:[[:space:],]\@="he=e-1
" syn match  rubySymbol   "[[:space:],{(]\%(\h\|[^\x00-\x7F]\)\%(\w\|[^\x00-\x7F]\)*[!?]\=:[[:space:],]\@="hs=s+1,he=e-1
 " hi def link rubyPredefinedConstant  rubyPredefinedIdentifier
" syn match rubyPredefinedConstant "\%(\%(^\|[^.]\)\.\s*\)\@<!\<\%(ARGF\|ARGV\|ENV\|DATA\|FALSE\|NIL\|STDERR\|STDIN\|STDOUT\|TOPLEVEL_BINDING\|TRUE\)\>\%(\s*(\)\@!"
" syn match rubyPredefinedConstant "\%(\%(^\|[^.]\)\.\s*\)\@<!\<\%(RUBY_\%(VERSION\|RELEASE_DATE\|PLATFORM\|PATCHLEVEL\|REVISION\|DESCRIPTION\|COPYRIGHT\|ENGINE\)\)\>\%(\s*(\)\@!"
```