When trying to launch the Unreal Engine on OS X Yosemite form the Epic Games Launcher, the editor will not give a warning saying that the current version of OS X is not compatible.
We were able to confirm that this message appears when attempting to open the Engine directly using the .app.
Result: The Unreal Engine will not launch with a warning stating that the engine cannot be ran with the current OS X version.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-40831 in the post.