Log Message:
Update ruby-sequel to 4.31.0.

= Improvements

* Sequel now works with ruby 2.3's --enable-frozen-string-literal,
  and all of the library files are set to use frozen string
  literals by default.

  A couple adapters and extensions depend on libraries that have
  issues with frozen string literals.  Pull requests have been sent
  to each of those dependencies.

* The migrators will now raise an exception if a migration file
  contains no migrations or more than one migration.

* The jdbc/postgresql adapter now supports using PostgreSQL specific
  types in bound variables.  Note that the current version of
  jdbc-postgres (9.4.1204) has regressions that affect this, users
  who need this support should stick with jdbc-postgres 9.4.1200 or
  below.

* The jdbc/postgresql adapter now works around a regression in Java
  method lookup in JRuby 9.0.5.0

* The setter methods added by the association_pks plugin now do type
  casting immediately, instead of right before the data will be used.
  This makes them more similar to column setter methods, and ensures
  that future calls to the getters that use cached values will
  return correctly typecast data.

* The PostgreSQL array parser in the pg_array extension now handles
  arrays with explicit bounds.  The explicit bounds are ignored, so
  such values do not round trip, and there is currently no support for
  creating arrays with explicit bounds.

* Creating a table with a simple non-incrementing primary key and a
  self-referential foreign key now works correctly on MySQL:

    DB.create_table!(:table) do
      Integer :id, :primary_key=>true
      foreign_key :fk, :table
    end

* Database#disconnect in the oracle adapter now works correctly on
  more recent versions of oci8 where #logoff can raise OCIException
  instead of OCIInvalidHandle.

= Backwards Compatibility

* The pg_array extension no longer defines
  Sequel::Postgres::PGArray::JSONCreator.  This should only affect
  backwards compatibility if you were accessing the constant directly.
  The :parser option to Sequel::Postgres::PGArray.register is also no
  longer respected, but that should not affect backwards compatibility.

* The Sequel::Model#convert_cpk_array private method that was added by
  the association_pks plugin has been removed.
  Sequel::Model#convert_pk_array handles both simple and composite
  primary keys now.