Version 0.1.15.

Fixed a garbage collector bug reported by Sean Ross. When the garbage
collector deletes object ids from the object table (because the
objects are dead and we may want to reuse their ids later for other
objects), it should also remove that object from the cache.  If it
doesn't, there's a possibility that the object id will be reused later
for a new object and the cache wil still refer to the old in-memory
object.