Upgrade bundler-audit from 0.4.0 to 0.5.0

Bundler Audit can now run check with the `—update` flag to update the
Ruby CVE repository before checking. This removes the need for two
separate commands in GitLab CI.

See the Changelog for more information:
https://github.com/rubysec/bundler-audit/blob/master/ChangeLog.md#050--2
015-02-28