lib/rockstar.js: make API URL configurable.

Instead of hard coding the github URL, we'll make it configurable.
This will help out down the road in the following situations:

  1) if we want to write a unit test, we can point the request
     at some sort of test server.
  2) if the API URL changes and this module isn't updated quickly
     enough, module users can just override the default URL.
  3) if a compatible API is offered by some other hosting provider,
     module users can override the github URL to point to the other
     service.