Update ruby-sequel to 4.19.0.

=== 4.19.0 (2015-02-01)

* Make jdbc/sqlanywhere correctly set :auto_increment entry in schema hashes (jeremyevans)

* Add Model#cancel_action for canceling actions in before hooks, instead of having the hooks return false (jeremyevans)

* Support not setting @@wait_timeout on MySQL via :timeout=>nil Database option (jeremyevans)

* Add accessed_columns plugin, recording which columns have been accessed for a model instance (jeremyevans)

* Use correct migration version when using IntegerMigrator with :allow_missing_migration_files (blerins) (#938)

* Make Dataset#union, #intersect, and #except automatically handle datasets with raw SQL (jeremyevans) (#934)

* Add column_conflicts plugin to automatically handle columns that conflict with method names (jeremyevans) (#929)

* Add Model#get_column_value and #set_column_value to get/set column values (jeremyevans) (#929)

(taca)