Scope the addressable dependency

Just for the sake of paranoia. I manually tested addressable against the original `0.1.0` release, the latest `0.1.x` release, the latest `1.x` release, and the latest `2.x` release. Addressable is apparently the most stable library ever created in Ruby, because they all worked (or this is just not using much of it! :grinning:).  So, canonical appears to work with all released versions of addressable, ever. But, I added a cap for a `3.x` release to at least ensure that it gets properly tested.