Regex match quotes

Make the shortcode parser capable of supporting both quote types.

Since Wordpress can create shortcodes with either or both types of quotes in the same shortcode, performing a regex match instead of string match has more flexibility if processing old shortcodes where you may not be confident of the format.

Specs updated ands should pass. Docs updated.

Version bump on the development gems and Ruby version, as well as fixing spec_helper if using Bundler as Rails was not included as a development dependency

Hope you like it!