Call `verify` to also adjust inventory_units

Removing Line Items won't adjust units without calling `verify`
Add logic that links units with line_items

Also added a migration for the required field
Set up testing environment
Add tests for OrderContents overrides
Add tests for `OrderContents#remove`
Add LineItem tests
Correctly nest contexts in order_contents_spec.rb
Add tests for OptionValue
Add tests for Variant
Filter dummy app when calculating coverage
Add tests for Product
Add tests for OptionType
Merge pull request #7 from 3drobotics/tests

Added tests and move to this gem the logic for attaching a line_item with an inventory_unit