Log Message:
Update ruby-sequel to 4.20.0.

=== 4.20.0 (2015-03-03)

* Restore the use of AUTOINCREMENT on SQLite (jeremyevans) (#965)

* Duplicate the associations hash when duplicating a model object (jeremyevans)

* Correctly apply association limit when eager loading with an eager block using \ 
default limit strategy on some databases (jeremyevans)

* Fix eager loading when using the :window_function limit strategy with an eager \ 
block and cascaded associations (jeremyevans)

* Add support for set_column_type :auto_increment=>true to add AUTO_INCREMENT \ 
to existing column on MySQL (jeremyevans) (#959)

* Add support for overridding the :instance_specific association option (jeremyevans)

* Recognize MSSQL bit type as boolean in the schema_dumper (jeremyevans)

* Skip eager loading queries if there are no matching keys (jeremyevans) (#952)

* Dataset#paged_each now returns an enumerator if not passed a block (jeremyevans)

* Use to_json :root option with string value as the JSON object key in the \ 
json_serializer plugin (jeremyevans)

* Allow create_enum in the pg_enum extension be reversible in migrations \ 
(celsworth) (#951)

* Have swift adapter respect database and application timezone settings (asppsa, \ 
jeremyevans) (#946)

* Don't have the static cache plugin attempt to validate objects (jeremyevans)

* Make freeze not validate objects if their errors are already frozen (jeremyevans)

* Only use prepared statements for associations if caching association metadata \ 
(jeremyevans)

* Set parent association when loading descendants in the rcte_tree plugin \ 
(jeremyevans)

* Add Database#transaction :before_retry option, specifying a proc to call \ 
before retrying (uhoh-itsmaciek) (#941)