Fix deprecated `:rubygems` directive in Gemfile
Use MultiJson instead of JSON gem
MultiJson is really great since it switches to the fastest JSON available. It is really common for gems to just use MultiJson (which uses Ruby's JSON by default) so consumers can include gems for faster JSON parsers at will. For example the `oj` gem is the fastest, but is a C extension, so folks using JRuby wouldn't want that. Anyway, MultiJson is really great. Hopefully you approve :)