Extra arg to edd_complete_download_purchase filter

This is addition to Software Licenses plugin so it can save order of
each download item in the cart
Changed $cart_order to $cart_index
Merge pull request #2776 from somewebmedia/enhacement/download-cart-order

Extra arg to edd_complete_download_purchase filter
Scrutinizer for code coverage
Update .scrutinizer.yml
Update .travis.yml
Update .travis.yml
Swap to use Scrutinizer for code coverage