Switch database cleaner strategy from `:trunction` to `:deletion`

:deletion performs better on Linux machines with slower I/O as it does a
lot less than `:trucation` according to:

https://stackoverflow.com/a/50075121/230526

spec/support/database_cleaner.rb

@@ -5,10 +5,12 @@ require_relative 'db_cleaner'
 RSpec.configure do |config|
   include DbCleaner
 
-  # Ensure all sequences are reset at the start of the suite run
+  # Ensure the database is empty at the start of the suite run with :deletion strategy
+  # neither the sequence is reset nor the tables are vacuum, but this provides
+  # better I/O performance on machines with slower storage
   config.before(:suite) do
     setup_database_cleaner
-    DatabaseCleaner.clean_with(:truncation)
+    DatabaseCleaner.clean_with(:deletion)
   end
 
   config.append_after(:context, :migration) do