Use `rake` to run the tests both locally and in CI

From http://about.travis-ci.org/docs/user/languages/ruby:

  > Travis will use Bundler to install your project's dependencies and
  > run rake by default to execute your tests. Please note that you
  > need to add rake to your Gemfile (adding it to just :test group
  > should be sufficient).

This DRYs up the copy-pasted bacon command -- I missed it in
.travis.yml when adding '-w' to the Rakefile, then couldn't figure out
why the warnings wouldn't show up on Travis...
Add a Gemfile so you can install dev dependencies with Bundler

(I'm trying to figure out how to install rip -- its website with the
standalone installation instructions is gone now... but it seems like
switching to Bundler + gemspec would be reasonable at this point?
Bundler is already used in .travis.yml, so this'll unify dependency
management between dev and CI, if nothing else...)

[20 min later] ... yeah, this isn't going well. I'm going to remove
deps.rip. RIP rip.

[25 min later] ... now that we have a Gemfile, the `bundle init` line in
.travis.yml is unnecessary. Travis will invoke Bundler automatically.