gb vendor update --all rewrite the repository url in the manifest file

[tinhtruong]

Some of our dependencies are hosted as private repositories on bitbucket, so to make gb work with those repositories, I have to manually change the repositories url in the manifest file to use ssh protocol instead of https.
But after running the gb vendor update --all, gb just changes all of the repositories url to https.
It would be great if gb can retain the repositories url in this case.

Cheers.