Log Message:
Upadte ruby-sequel to 4.26.0.

=== 4.26.0 (2015-09-01)

* Make Dataset#== not consider frozen status in determining equality (jeremyevans)

* Support :if_exists option to drop_column on PostgreSQL (jeremyevans)

* Add Dataset#grouping_sets to support GROUP BY GROUPING SETS on PostgreSQL \ 
9.5+, MSSQL 2008+, Oracle, DB2, and SQLAnywhere (jeremyevans)

* Fix handling of Class.new(ModelClass){set_dataset :table} on ruby 1.8 (jeremyevans)

* Use range function constructors instead of casts for known range types in \ 
pg_range (jeremyevans) (#1066)

* Make class_table_inheritance plugin work without sti_key (jeremyevans)

* Detect additional disconnect errors when using the tinytds adapter (jeremyevans)

* Make offset emulation without order but with explicit selection handle \ 
ambiguous column names (jeremyevans)

* Allow preparing already prepared statements when emulating limits and/or \ 
offsets (jeremyevans)

* Have Sequel::NoMatchingRow exceptions record the dataset related to the \ 
exception (pedro, jeremyevans) (#1060)