Update ruby-sequel to 5.0.0.

=== 5.0.0 (2017-09-01)

* Make bin/sequel -M option always use base 10 (jeremyevans)

* Don't use savepoints when creating indexes inside a transaction on databases
  that don't support transactional schema modifications (jeremyevans) (#1407)

* Support :if_not_exists option when creating indexes on PostgreSQL 9.5+
  (DyegoCosta) (#1405)

* Make threaded connection pools not block while connections are being made
  (jeremyevans)

* SQL::Expression#clone and #dup now return self, since all expressions should
  be frozen value objects (jeremyevans)

* Don't create empty arrays for unused association callbacks (jeremyevans)

* Cache association method name symbols instead of recomputing them everytime
  (jeremyevans)

* Raise an exception if attempting to create a prepared statement using a
  dataset with a delayed evaluation (jeremyevans)

* Make ConnectionPool#size thread safe by using the pool mutex (jeremyevans)

* Use instance_exec instead of instance_eval when passing a block, to work
  with lambdas that accept no arguments (jeremyevans)

* Freeze SQL::StringAgg instances in string_agg extension (jeremyevans)

* Freeze SQL::DateAdd instances in date_arithmetic extension (jeremyevans)

* Freeze SQL::Expression.comparison_attrs (jeremyevans)

* Rename SQL::Subscript#f to #expression, keeping #f as an alias (jeremyevans)

* Require the :pool_class Database option be a class to use a custom
  connection pool (jeremyevans)

* Make the class_table_inheritance plugin raise an Error during update if any
  UPDATE query does not affect a single row (jeremyevans)

* Change most send calls to public_send unless calling private methods is
  expected (jeremyevans)

* Database schema and schema generator methods now return nil (jeremyevans)

* Model#validates_unique in the validation helpers plugin now defaults to only
  checking on new or modified values (jeremyevans)

* Deprecate Model#_before_validation (private_method), use
  Model#before_validation now (jeremyevans)

* Always run before/after/around validation hooks when saving, even when not
  validating the object (jeremyevans)

* Deprecate Model use_after_commit_rollback class and instance accessors
  (jeremyevans)

* Deprecate Model.allowed_columns reader (jeremyevans)

* Freeze internal constants that shouldn't be modified at runtime (jeremyevans)

* Attempt to connect to the database immediately when creating the Database
  instance (jeremyevans)

* Make association_pks plugin delay the setting of associated objects until
  the current object is saved by default (jeremyevans)

* Joined datasets used as model datasets are now automatically wrapped in a
  subquery (jeremyevans)

* Setting an invalid dataset for a model class now raises an exception by
  default (jeremyevans)

* Getting all values for newly created models now happens before calling
  after_create, instead of after (jeremyevans)

* Remove use of @was_new/@columns_updated instance variables when saving model
  objects (jeremyevans)

* Disable symbol splitting by default (jeremyevans)

* Make datasets frozen by default (jeremyevans)

* Drop support for ruby 1.8.7, minimum now is 1.9.2 (jeremyevans)

* Remove deprecated adapters, extensions, plugins, constants, and features
  (jeremyevans)

(taca)