[EE] Upgrade shoulda-matchers to 4.0.1
This removes the warning when using Ruby 2.6:
BigDecimal.new is deprecated; use BigDecimal() method instead.
This also adds Rails 5 support, which eliminates the need for the monkey patch to handle https://github.com/thoughtbot/shoulda-matchers/issues/913.
CE port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28443
Edited by Stan Hu