Add Ruby 1.8.7 compatibility

I’d like to use Naught to replace [`Twitter::NullObject` in the `twitter` gem](https://github.com/sferik/twitter/blob/master/lib/twitter/null_object.rb), however, the current version of the gem still supports Ruby 1.8.7 and I don’t want to bump the major version just to add Naught.

I also took this opportunity to add other Ruby versions and implementations to the Travis CI configuration (JRuby, Rubinius, etc.).

Do you have any objection to supporting Ruby 1.8.7, [at least through June 2014](https://www.ruby-lang.org/en/news/2013/12/17/maintenance-of-1-8-7-and-1-9-2/)?