def tags_newer(self, versions_file, majors):
        """
        Checks this git repo tags for newer versions.
        @param versions_file: a common.VersionsFile instance to
            check against.
        @param majors: a list of major branches to check. E.g. ['6', '7']
        @raise RuntimeError: no newer tags were found.
        @raise MissingMajorException: A new version from a newer major branch is
            exists, but hasn't been downloaded due to it not being in majors.
        """
        highest = versions_file.highest_version_major(majors)
        all = self.tags_get()

        newer = _newer_tags_get(highest, all)

        if len(newer) == 0:
            raise RuntimeError("No new tags found.")

        return newer