Incorrect sorting in the calculation of the discount

[icurdinj]

A user reported: "This strange behavior is due to a change in the work function uasort. I described it in detail on the SO: http://stackoverflow.com/questions/34281113/the-different-behavior-of-the-function-uasort-in-php-5-5-and-php-7-0"
Be warned until I get around to checking & fixing this.
If someone fixes this before, pull requests are welcome.

[icurdinj]

Fix merged from pull request, awaiting testing results.

[ed29]

This works for php7:facebook/hhvm#2416

[icurdinj]

Seems fixed.