Log Message:
Update sequel to 4.28.0.

=== 4.28.0 (2015-11-02)

* Add boolean_subsets plugin, which adds a subset for each boolean column \ 
(jeremyevans)

* Add subset_conditions plugin, which adds a method for each subset returning \ 
the filter conditions for the subset (jeremyevans)

* Make the list plugin work better with the auto_validations plugin when there \ 
is a validation on the position column (jeremyevans)

* Make to_csv for model datasets call instance methods, just like Model#to_csv, \ 
in the csv_serializer plugin (skrobul) (#1088)

* Raise Sequel::NoExistingObject instead of generic error if Model#refresh can't \ 
find the related row (jeremyevans)