Cannot open Aperture 3.5.1 in Yosemite

Since upgrading to Yosemite 10.10.3 (including Photos) Aperture 3.5.1 will no longer open. I get the message "In order to open Aperture.app, you need to update to the latest version." However, when I search the Mac App Store I get a message saying the item I searched for is not available in the US App Store. Is there a way to continue to use Aperture?