Log Message:
Update ruby-sequel to 4.39.0.

=== 4.39.0 (2016-10-01)

* Make active_model plugin use rollback_checker instead of after_rollback hook \ 
(jeremyevans)

* Add Database#rollback_checker, which returns a proc that returns whether the \ 
in progress transaction is rolled back (jeremyevans)

* Add Sequel::Database.set_shared_adapter_scheme to allow external adapters to \ 
support the mock adapter (jeremyevans)

* Make hook_class_methods plugin not use after commit/rollback model hooks \ 
(jeremyevans)

* Support add_column :after and :first options on MySQL (AnthonyBobsin, \ 
jeremyevans) (#1234)

* Support ActiveSupport 5 in pg_interval extension when weeks/hours are used in \ 
ActiveSupport::Duration objects (chanks) (#1233)

* Support IntegerMigrator :relative option, for running only the specified \ 
number of migrations up or down (jeremyevans)

* Make the touch plugin also touch associations on create in addition to update \ 
and delete (jeremyevans)

* Add :allow_manual_update timestamps plugin option for not overriding a \ 
manually set update timestamp (jeremyevans)

* Add Sequel.[] as an alias to Sequel.expr, for easier expression creation \ 
(jeremyevans)

* Add PostgreSQL full_text_search :to_tsquery=>:phrase option, for using \ 
PostgreSQL 9.6+ full text search phrase searching (jeremyevans)

* Add JSONBOp#insert in pg_json_ops extension, for jsonb_insert support on \ 
PostgreSQL 9.6+ (jeremyevans)

* Support add_column :if_not_exists option on PostgreSQL 9.6+ (jeremyevans)