Allow detecting repository on GitHub Enterprise

[mrmanc]

Alters the regex that extracts the org/username and repo name from the repo address based on suggestion by @benbalter, so that it can cope with repos that do not exist on github.com (e.g. GitHub Enterprise).

Adds a test for this change.

Resolves #120

[mrmanc]

In addressing why I was getting local failures (#148) I found that the rspec expects looked wrong here, and have resolved this. I think I have also resolved #148. Would you be open to me adding those changes to this PR or would you prefer I open a new one? There’s a little overlap as I’ve copied the inaccurate expects convention in the commits for this PR already, and I’d like to tidy that up.

[parkr]

I believe the AppVeyor failure is a flake (a failed network connection, perhaps?).

Let's ship this as-is, and add further changes in a separate PR? Feel free to ping me directly, @mrmanc. Thank you!

@jekyllbot: merge +minor

[mrmanc]

Awesome, thanks for this. I’ll sort out a separate PR shortly. Apologies for pinging Ben—I thought it best as he’d suggested the fix. I’ll be sure to aim it at you next time :)