In Arel 7.0.0 (Arel 7.1.4 is used in Rails 5.0) the `engine` parameter
of `Arel::UpdateManager#initializer` was removed.

This commit makes the gitlab database helpers work both in rails 4 and
rails 5.
Fixes errors like this one:

```
1) Gitlab::Database::MigrationHelpers#update_column_in_batches when running outside of a transaction updates all the rows in a table
    Failure/Error:
      update_arel = Arel::UpdateManager.new(ActiveRecord::Base)
        .table(table)
        .set([[table[column], value]])
        .where(table[:id].gteq(start_id))

    ArgumentError:
      wrong number of arguments (given 1, expected 0)
    # ./lib/gitlab/database/migration_helpers.rb:317:in `new'
    # ./lib/gitlab/database/migration_helpers.rb:317:in `block in update_column_in_batches'
    # ./lib/gitlab/database/migration_helpers.rb:307:in `loop'
    # ./lib/gitlab/database/migration_helpers.rb:307:in `update_column_in_batches'
    # ./spec/lib/gitlab/database/migration_helpers_spec.rb:367:in `block (4 levels) in <top (required)>'
```