The ASA w/ Firepower does not support the blocking of https traffic. Cisco is supposed to release support for blocking https traffic this summer. However, enabling this feature will put a heavy load on your ASA and the fact that it's a 5515x, it may bring it to a crawl. If you have a small number of users, like less than 50, it may be ok to do https url filtering. Any more than that, you would either need a larger ASA or an Ironport WSA.
The Ironport WSA already supports https url filtering and is purpose built for this feature.
