Be sure to grab the right AP image from http://support.arubanetworks.com. Only the last version supports 802.11ac.
Orion: RAP3, IAP92/93,IAP104/105, IAP175
Cassiopeia: IAP134/135
Pegasus: RAP108/109, IAP114/115
Aries: RAP155/155P
Ardmore: IAP224/225 (802.11ac)