Loosen/update dependencies for Rack, Addressable, etc.

The update to Rack ~> 1.5 removed a spec, since the Rack spec itself has
changed. Rack no longer requires a Response to have the Content-Type
header.

It's likely other spec changes to accomodate the most recent Rack should
occur.
Update CI to test Ruby 2.0.0 (and remove some others)