Test Updates

This PR includes a number of changes to get specs running and to allow the proper use of bundler (e.g. `bundler console`) for this gem.  Changes include:

1. Removing the Gemfile.lock and adding it to .gitignore
2. Moving development dependencies from the Gemfile to the ruby-ip.gemspec, and updating the Gemfile to reference the gemspec
3. Some minor changes to the gemspec to bring it up to date.
4. Upgrading from TestUnit to Minitest, and all related changes to the specs to make them green.
5. Adding a .travis.yml file, that can be used in conjunction with Travis CI.

There are no changes to the gem's behavior included in this PR.

After this PR is applied I'm able to get all specs running locally with Ruby 2.1.2.