Small changes to run on Ruby > 1.8.7

* Added a `Gemfile` to use bundler in development (it was useful when switching rubies to test)
* Added a `rake` task to run the tests
* Fixed a failing test in Ruby > 1.8.7

(Note: this is the first of a series of 3 PR; I'm creating them separately in case you don't want to merge them all)