GitHub Enterprise URLs not supported in private repositories

[bgreenlee]

I've developed a package for internal use at my company, and at some point in the past, the ability to install from a private repository broke. In the console I see:

Package Control: Invalid "details" value "https://github.mycorp.com/myrepo" for one of the packages in the repository https://github.mycorp.com/raw/bgreenlee/sublime-channel/master/repository.json.

I just dug in and looked at the code in github_client.py and it's all hard-coded to use public github urls, which is why it is breaking.

I'm happy to work on a fix, although it might be a while before I can get to it, so I'm filing this issue for now.