resolves #138: git-pr

@kevgo @allewun 
If I give an invalid protocol, it still returns the SSH version. Its better practice and more robust to explicitly check against "SSH" here, and use the else clause to throw an exception. Yes, that isn't really necessary as of yet, but it makes the code base more robust when changing something later, and removes the need to debug around to find this place if we ever add another protocol.