Fix malfunctioning double order plugin

[Zachary Alig]

Original Request:

Double orders – a plugin was used to prevent users from adding two of the same item to the cart. However, an update caused this plugin to break the checkout process and was disabled. We need to resolve this issue (which now could be as simple as making sure all plugins are up-to-date)

[Zachary Alig]

Hypothesis:

Orange "Add to Cart" buttons on http://roofingintel.staging.wpengine.com/buy/ are missing an attribute.

Screenshot:

[Zachary Alig]

Resolved in commit 20b9451