Jruby compat

This branch contains the following changes to the gem:

* Updating the gemspec to grab the geokdtree gem instead of kdtree.
* Modification of City class to use geokdtree method calls and loading files.
* Modification of specs to use new method calls/class expectation because of gem changes.

To test:

* Pull repo down and cd to the directory
* Run `bundle install` to install all of the gem's dependencies
* Run `gem install nearest_time_zone` to locally install the gem
* Run `irb` and type `require nearest_time_zone` to require the gem
* The api for this gem runs like `NearestTimeZone.to(lat, long)`