Log Message:
databases/ruby-sequel: update to 5.6.0

=== 5.6.0 (2018-03-01)

* Dedup :db_type strings in schema hashes on Ruby 2.5+ (jeremyevans)

* Make schema_caching extension work with :callable_default schema values \ 
(jeremyevans)

* Freeze string valuse in hashes returned by Database#schema when using the \ 
schema_caching extension (jeremyevans)

* Protect migration file loading with a mutex to not break when multiple threads \ 
load migration files simultaneously (jeremyevans)

* Respect identifier mangling rules when renaming columns on Microsoft SQL \ 
Server (jeremyevans)