Enable merge queue on the `hashbrown` repository

To use them instead of bors. See discussion [here](https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/Reducing.20the.20amount.20of.20repos.20that.20use.20bors.2Fhomu/near/476384370).

- https://github.com/rust-lang/team/pull/1577
- https://github.com/rust-lang/hashbrown/pull/575