Support Phoenix 1.5

I have a Phoenix 1.4 project that I tried upgrading to use Phoenix 1.5, but drab holds this back because Phoenix 1.5 requires Phoenix PubSub 2.0, while drab still depends upon `phoenix_pubsub ~> 1.0`.

Start by locating drab's phoenix_pubsub dependency declaration and checking the compatibility constraints for Phoenix 1.5 and PubSub 2.0. Update the dependency so the project can use the required PubSub version, then run the existing test suite and confirm Phoenix 1.4 compatibility is not unintentionally lost.

Written by the indexing model from the issue text.