update danger to check for Approved

We've started using Approve rather than LGTM to mark merge requests as ready to merge, but danger will still report:

elif "LGTM (Merge OK)" not in mr_labels:
    warn(
        "This merge request is currently in review. "
        "It should not be merged until it is marked with the *LGTM* label."
    )

I'd fix it myself, but I'm so unfamiliar with the gitlab API, I don't even know how Mr. Labels got set in the first place.