Log Message:
databases/ruby-arel-helpers: update to 2.9.1

Update ruby-arel-helpers to 2.9.1.

2.9.1

*  Remove has_rdoc from gemspec.

2.9.0

* Add support for Rails 6.

2.8.0

* Add support for Rails 5.2.1, which changed the arity of several internal
  methods that we shamelessly use.

2.7.0

* Add support for Rails 5.2.