Update mutant dependencies
Add support for MemoryScope#where with no arguments
Add support for MemoryScope#not
Add setup for database_cleaner in core

* Cleanup test DB prior each suite run for reproducible results
Add optional database cleaning transaction around examples
Change CreateItemAdjustmentsSpec to be executed isolated
Kill relevant mutations in CreateItemAdjustments

* There is one semantically not relevant mutation left that fixes a
  performance problem.