I am not really sure why the tests in Travis CI are failing. Is it my error or is it a configuration problem?

In any case, further suggestions for improvements are welcome.

I also would like to ask you about your opinion of the new implementation of
func requestUserVerificationCode(scope string) (map[string]string, error).
It returns a map[string]string which holds 4 values originating from the response of the Github API. I would like to use all the values but do not know what would be a clean way to return all of them.

The tests fail because of:

bridge/github/config.go:219:25: Errorf call has arguments but no formatting directives
bridge/github/config.go:260:25: Errorf call has arguments but no formatting directives

Easy to fix

I also would like to ask you about your opinion of the new implementation of
func requestUserVerificationCode(scope string) (map[string]string, error).
It returns a map[string]string which holds 4 values originating from the response of the Github API. I would like to use all the values but do not know what would be a clean way to return all of them.

Returning a map is not great indeed. You can either make up your own struct with 4 fields to return those values or return 4 values + an error as go allow you to.

I have a question since I am not very familiar with Github: Can I rebase my branch now onto your current master and force-push into my fork? I am wondering if it would create problems in this pull request.

Rebase and push --force is the correct way. It sometimes get github a bit confused if it can't reattach the comments at the correct place but that's not a bit deal. Just don't do it to often. Rebasing to have the updates from master is a very valid reason to do that.

Rebase completed @MichaelMure.

Well, that's awesome, thank you!
FYI, I cleaned it up just a little bit with 9daa8ad, notably the defer in the loop (defer trigger when the function return, not after each loop, which means the bodies were accumulating in memory).