Log Message:
Update ruby-arel to 6.0.0.

=== 6.0.0 / 2014-11-25

* Enhancements

  * Remove deprecated `Arel::Expression`
  * Remove deprecated `Arel::SqlLiteral`
  * Remove deprecated `SelectManager#joins`
  * Remove deprecated `SelectManager#to_a`
  * Remove deprecated `Arel::Sql::Engine`
  * Remove deprecated `Arel::InnerJoin` constant
  * Remove deprecated `Arel::OuterJoin` constant

== 5.0.0 / 2013-12-04

* Enhancements

  * Remove deprecated code

* Bug Fixes

  * Fix serializing a relation when calling `to_yaml`

=== 4.0.2 / 2014-02-05

 * Bug Fixes

  * Fix `SqlLiteral` YAML serialization
  * PostgreSQL bugfix for invalid SQL in subqueries

== 4.0.1 / 2013-10-22

* Enhancements

  * Cache visitor dispatch on a per-visitor basis
  * Improve performance of #uniq across a large number of nodes

* Bug Fixes

  * Make visitors threadsafe by removing @last_column
  * Support `columns_for_distinct` with Oracle adapter