Add development dependencies to the gemspec

This PR adds development dependencies to the gemspec, so future contributors don't have to play whack-a-mole with adding dependencies.

You may want to consider in the future managing your gemspec by hand instead of using echoe. There is [no need to generate gemspecs](http://yehudakatz.com/2010/04/02/using-gemspecs-as-intended/) with alternative tools, and it makes dependency resolution a PITA when the Gemfile uses the gemspec as your source, but the gemspec is being generated by a tool that's not already in the gemspec. But, for now, this will suffice.