Revert "Use Arel instead of String for AR Enumerator conditionals"

[pedropb]

Reverts #456

#456 introduced a bug in the following code path:

job-iteration/lib/job-iteration/active_record_cursor.rb

Line 37 in e17e2ca

@base_relation = relation.reorder(@columns.join(","))

when relation.reorder(*<array_of_arels>) is invoked, implicitly a to_s is called on each Arel, which expands them to "table.column" and ActiveRecord prepends table again, leading to a clause like:

 ORDER BY `products`.`products.created_at`, products`.`products.id`

instead of:

 ORDER BY products.created_at, products.id

which was the previous behaviour.