bump version to 2.3.9

Merge pull request mbleigh#125 from mnaberez/postgresql_version

Restore support for PostgreSQL with Rails versions before 5.0.0

bump version to 2.3.7

bump version to 2.3.6 with Rails 5 support

Update CHANGELOG.md

bump version to 2.3.4

bump version to 2.3.2

bump version to 2.3.2

bump version to 2.3.1

Bump version to 2.2.0