Upgrade mutant gem dependencies
Fix specs to kill all alive mutations in Spree::OrderContents#add
Fix Spree::OrderContents#add to not save deleted line items

* The line items, which may be deleted, do not respond well to having
  validations run, since the before_validation hook modifies the object.
  Any deleted items are frozen, so this will raise an exception.
Merge pull request #40 from MountainRoseHerbs/2-3-stable-mrh-fix-order-contents-add

Fix Spree::OrderContents#add to handle deleted line items properly