Fix approvals check in GitLab community edition

See https://github.com/smarkets/marge-bot/issues/258 and https://github.com/geraldcombs/marge-bot/commit/0803806a86c192cc2620ba980709b1ed337dae26 and also #2.

Here's an example where the marge-bot erroneously reports fails to merge due to lacking approvals: tezos/tezos!5147 (merged)