Github Enterprise fix #79

Github cache builder is now configurable so you can also point to your own Github Enterprise server. If you do not specify any of the new config options, the builders' behaviour is exactly the same as before.

For auto_paginate to work with Github Enterprise an additional fix is needed in the Octokit.rb package. A patch for that is already supplied to Octokit.rb (https://github.com/octokit/octokit.rb/pull/440)