Test against Ruby 1.9.2, 1.9.3, and 2.0.0

* Use secure Rubygems URL to avoid Bundler warnings like:

> The source :rubygems is deprecated because HTTP requests are insecure.
> Please change your source to 'https://rubygems.org' if possible, or
> 'http://rubygems.org' if not.

* Add Travis image to README near support information.