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)>'
```
