Log Message:
Update databases/ruby-sequel to 1.5.0.  Changes from version 1.4.0 include:

* No longer depend on the assistance gem, merge the Inflector and
  Validations code

* Make the validation errors API compatible with Merb
* Add validates_uniqueness_of, for protecting against duplicate entries
  in the database

* Use reciprocal associations for all types of associations in the
  getter/setter/add_/remove_ methods
* Fix many_to_one associations to cache negative lookups
* Add eager loading of associations via new sequel_core object graphing
* Fix many_to_many associations with classes inside modules without an
  explicit join table

* Add the eager! and eager_graph! mutation methods to model datasets
* Add Model.def_dataset_method, for defining methods on the model that
  reference methods on the dataset
* Change Model#=== to always be false if the primary key is nil
* Add Model#hash, which should be unique for a given class and primary
  key (or values if primary key is nil)
* Make Model#reload clear any cached associations
* Fix Model#destroy so that it returns self, not the result of after_destroy
* Make Model#initialize, Model#set, and Model#update_with_params
  invulnerable to memory exhaustion
* Add Model.str_columns, which gives a list of columns as frozen strings