Update ruby-sequel to 4.40.0.

=== 4.40.0 (2016-10-28)

* Make column_select plugin not raise an exception if the model's table does not exist (jeremyevans)

* Make dataset_associations plugin correctly handle (many|one)_through_many associations with single join table (jeremyevans) (#1253)

* Add s extension, with adds Sequel::S module that includes private #S method for calling Sequel.expr, including use as refinement (jeremyevans)

* Add symbol_as and symbol_as_refinement extensions so that :column.as(:alias) is treated as Sequel.as(:column, :alias) (jeremyevans)

* Add symbol_aref and symbol_aref_refinement extensions so that :table[:column] is treated as Sequel.qualify(:table, :column) (jeremyevans)

* Add Sequel.split_symbols=, to support the disabling of splitting symbols with double/triple underscores (jeremyevans)

* Make SQL::QualifiedIdentifier convert SQL::Identifier arguments to strings, fixing Sequel[:schema][:table] usage in schema methods (jeremyevans)

* Do not attempt to combine non-associative operators (jeremyevans) (#1246)

* Automatically add NOT NULL to columns when adding primary keys if the database doesn't handle it (jeremyevans)

* Make prepared_statements plugin correctly handle lookup on joined datasets (jeremyevans) (#1244)

* Make Database#tables with :qualify=>true option handle table names with double underscores correctly (jeremyevans) (#1241)

* Add SQL::Identifier#[] and SQL::QualifiedIdentifier#[] for creating qualified identifiers (jeremyevans)

* Add support for Dataset#insert_conflict :conflict_where option, for a predicate to use in ON CONFLICT clauses (chanks) (#1240)

* Freeze Dataset::NON_SQL_OPTIONS, add private Dataset#non_sql_options, fixing thread safety issues during require (jeremyevans)

* Make the callable returned by Database#rollback_checker thread safe (jeremyevans)

* Make lazy_attributes and dataset_associations plugins work if insert_returning_select plugin is loaded before on model with no dataset (jeremyevans)

(taca)