Allow for private prefixes to method definitions

- Addresses this comment: https://github.com/atom/language-ruby/issues/7#issuecomment-67577883
- Taken from ruby.tmbundle: https://github.com/textmate/ruby.tmbundle/commit/254891d847cf26eb0e90098600e3f582b38665a6
I think you want it to be `\\s+` instead, so it matches `private def`, but not `privatedef`