Fix specs to pass with tighter constraints

* Change to use factory girl when the object used would otherwise be
  invalid.
Fix test_app rake task to create a dummy app for the sample directory
Fix code formatting
Remove unused method from Spree::OrderUpdater
Refactor Spree::OrderUpdater specs to use an explicit subject
Refactor Spree::OrderUpdater#persist_totals to be simpler
Add validations to Spree::Order

* This focuses most on improving the existing validations and adding
  validations for the totals that are updated based on user input.
Change Spree::OrderUpdater#persist_totals to validate the totals

* If the totals are greater than the database columns can store, then we
  should avoid hitting the database and report the error.
Merge pull request #30 from MountainRoseHerbs/validate-totals

Validate totals
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
Fix style to ease next changes
Kill relevant mutations in CreateItemAdjustments

* There is one semantically not relevant mutation left that fixes a
  performance problem.
Use in memory state to create adjustments

* Reduces transitive lazy loads.
* Refactors away last alive mutation without introducing a performance
  regression.