Release 4.0.0 (#339)

Bump to 4.0.0, closes #336

Merge branch 'jhawthorn-patch-1'

Update to currently supported ruby versions (#323)

* Update to currently supported ruby versions

* Fix the circle yml

* Change to always test the latest releases of 2.4, 2.5, 2.6

Merge branch 'sunbirddcim-Use_skip_locked'

Version 3.2.0.RC1

Bump version to 3.1

💎 bump version (3.1 RC1!!!) 🎉

relax `pg` dependency to allow both `0.17.x` and `0.18.x`. Closes #240.

/cc @jipiboily

Merge branch 'fix-upgrade' into 3-0-stable

bump version