Bump to version 2.1.0

Changes in bump:

* [Toggle feature as a boolean](https://github.com/FetLife/rollout/pull/65)
* [Added Rollout#clear! to remove feature data](https://github.com/FetLife/rollout/pull/63)
* [#id_user_by added](https://github.com/FetLife/rollout/pull/60)
* [Make redis a dev dependency](https://github.com/FetLife/rollout/pull/55)

closed #68