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!
I got used to doing it this way for practical reasons some time ago. Like when one really had to support 1.8 and 1.9, and you needed different debuggers per version, it was a lot easier to just use the `platforms` option from bundler. In the present day where everyone uses bundler I don't quite understand what's the use case for setting these type of dependencies in the gemspec.

I'm not sure how strongly I feel about this :P