I noticed this, too (v2.77b). Having done a small amount of digging around, it seems that the Bone Layer Management add-on can be downloaded independently - though whether it works with the latest versions of Blender, I don't know.
*sigh* Didn't finish writing my post (darned ENTER = POST!) ^_^ What I was GOING to say was that there is, however, quite a lot of active discussion taking place regarding an overhaul of Blender's core layer management system - so if you or anyone else can put up with the existing system for now, you may find that preferable to going to the trouble of spending however-long getting a possibly soon-to-be redundant add-on working. Read more, here: https://developer.blender.org/T38384
