Keep the Gemfile alphabetical

This commit brought to you by OCD Anonymous.
Don't bother using require_relative in specs

RSpec adds /lib to LOAD_PATH by default, so require works fine.
Explicitly include Rack in Gemfile
Drive out basic implementation of Bouncer#call

This assumes that Host and Mapping support the Active Record 4.0 query
interface, which is likely to be the case once they're implemented.
Extract shared examples and introduce a context for archived URLs

This is in preparation for introducing more functionality (i.e.
actual redirects).
Specify the kind of response for archived URLs
Add support for redirects