Corrects transactional fixture check for Rails 5
Corrects transactional fixture check for Rails 5

In Rails 5 `ActiveSupport::TestCase` now uses `use_transactional_tests`
rather than `use_transactional_fixtures` as its method to check whether
transactional fixtures are being utilized for the current test.

See: https://github.com/rails/rails/pull/19282

kenn/slavery