Modernized time-warp a bit with bundler

/cc @bjhess @lorenzoplanas @tjschuck 

* Added bundler so we can build and release new gem versions with it. 
* Also using Minitest 5... because.
* Included a mix of #9 and #7 so that it works with ruby 1.9's `Time.new`.

Once merged we can release a new version.

Cheers!
Why not also add these to the gemspec as development dependencies?  Keep everything in one place.