Vendorize gem dependencies

* Use bundle install --path=vendor to regenerate dependencies
* Modify the `LOAD_PATH` so that the vendorized versions of the
  dependencies are found.
Add homebrew formula

* remove gemspec
* move dependencies to Gemfile
* Add formula for Homebrew (will be moved to thoughtbot/formula when
  released)
Move critical load path tweaking to liftoff.rb

We want to make sure this load path stuff happens for specs, too, so we
should do it in a central location that isn't dependent on the
executable.
Add spec helper
Clean up rake tasks