Decouple from Shipr.

Just some prep work for a new version of Shipr (which is being rebranded as Tugboat). This makes the plugin more generic and only talks to GitHub. The only reason it was talking to Shipr directly was to install the GitHub hook automatically, but I don't actually want to do that anymore to make it easier to phase in the new version (also faster feedback in chat since there's 2 less round trip http requests).