Use Travis CI service

This pull requests adds a Travis CI configuration file which will run all the tests for the project against 1.9.2, 1.9.3, 2.0.0, and Rubinius each time commits are pushed to the project.

The service hook for Travis CI will need to be set up for the project once this pull request is merged, see [Travis CI - Getting started](http://about.travis-ci.org/docs/user/getting-started/) for details.

1.8.7 and REE have been left out because they all fail their tests with the same problem as has already been reported in #139

There is also a commit in this pull request to fix the gemspec (which covers the same ground as #146)