databases/ruby-sequel update to 5.18.0

=== 5.18.0 (2019-03-01)

* Use singleton .call methods on plain objects instead of procs/methods for faster type conversion (jeremyevans)

* Add Sequel::SQL::Blob.call to avoid indirection when converting values from the database (jeremyevans)

* Use while instead of each for inner loops in sqlite and jdbc adapters for better performance (jeremyevans)

* Make after_initialize plugin not make the argument to Model.call optional (jeremyevans)

* Allow Dataset#paged_each to be called without a block in the postgres and mysql2 adapters (jeremyevans)

* Remove flow-control exceptions in connection_expiration and connection_validator extensions (jeremyevans)

* Add throw_failures plugin for throwing ValidationFailed and HookFailed exceptions instead of raising them, up to 10x performance increase on JRuby (jeremyevans)

* Support tzinfo 2 in addition to tzinfo 1 in the named_timezones extension (jeremyevans) (#1596)

(taca)