Revert "Update unsecure dependencies"

This reverts commit 6c7e9c50 (see !38 (merged)).

The problem is that changes to the OAuth gem made our OAuth integration break on staging.

E.g. logging in via https://gitlab-org-gitlab-services-version-gitlab-com-staging.version-staging.gitlab.org/users/sign_in is broken.

I chose to completely revert the commit to unblock deploying to production as soon as possible.