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
Don't bother naming the Host double

Once it's been stubbed we can just refer to it by its global name.
Give the path hash double a name
Explicitly require Rack::Request, not all of Rack