elm-upgrade fails if the repository URL starts with http instead of https

[pdamoc]

The repository I tried this on had a default http://github.com/user/project.git value. (I'm guessing that this is what the default value might have been on elm version that was uses to start the project).

The culprit is this line.

My guess is that it would also fail if the repository field is a valid repository but not on github.