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!
By the way, this is the recommended way to [install different gems depending on the ruby version](http://en.wikibooks.org/wiki/Ruby_Programming/RubyGems#How_to_install_different_versions_of_gems_depending_on_which_version_of_ruby_the_installee_is_using). I don't think you'd want me to do that.

So we can only reasonably move the `minitest` gem to the gemspec.

I think the pattern nowadays is to use the `Gemfile` for development dependencies and the `gemspec` for the rest. For example, [rails does](https://github.com/rails/rails/blob/master/rails.gemspec) it [this way](https://github.com/rails/rails/blob/master/Gemfile).