Explicit gemspec ; use bundler/gem_tasks

I was playing around with my checkout of nugget and noticed three things:

* I could not do a 'bundle install' to get all of the deps on my machine
* I could not do a 'bundle exec nugget' to test the development version of nugget
* I could not reference a specific sha of nugget in my Gemfile since there was not a `gemspec` file in the root of the repo

This PR shifts things around to make these things possible.  The gemspec should be the same as what was in the `Rakefile` before.

If you know of a better way to accomplish the above, I'm all ears.  This is working for me right now in https://github.com/gorsuch/litterbox.

:v: