Attempt to fix random CI failures

We get frequent random CI failures with errors on feature specs, like the following:

Failures:

  1) Search admin searches with an unknown filter
     Failure/Error: DatabaseCleaner.clean

     ActiveRecord::StatementInvalid:
       PG::TRDeadlockDetected: ERROR:  deadlock detected
       DETAIL:  Process 12270 waits for AccessExclusiveLock on relation 16412 of database 16386; blocked by process 12192.
       Process 12192 waits for AccessShareLock on relation 16400 of database 16386; blocked by process 12270.
       HINT:  See server log for query details.
       : TRUNCATE TABLE "public"."blog_posts", "public"."series", "public"."countries", "public"."log_entries", "public"."payments", "public"."product_meta_tags", "public"."customers", "public"."products", "public"."orders", "public"."line_items" RESTART IDENTITY CASCADE;
     # ./spec/support/database_cleaner.rb:19:in `block (2 levels) in <top (required)>'
     # ------------------
     # --- Caused by: ---
     # PG::TRDeadlockDetected:
     #   ERROR:  deadlock detected
     #   DETAIL:  Process 12270 waits for AccessExclusiveLock on relation 16412 of database 16386; blocked by process 12192.
     #   Process 12192 waits for AccessShareLock on relation 16400 of database 16386; blocked by process 12270.
     #   HINT:  See server log for query details.
     #   ./spec/support/database_cleaner.rb:19:in `block (2 levels) in <top (required)>'

Finished in 29.41 seconds (files took 3.65 seconds to load)
418 examples, 1 failure

This may or may not be related to DatabaseCleaner. As an attempt to fix it, I'm trying this configuration, copied straight from their documentation at https://github.com/DatabaseCleaner/database_cleaner/blob/v1.7.0/README.markdown