Upgrading to latest roaring.

The toImmutableRoaringBitmap method is unnecessary and buggy. Version 0.4.2 removes it. Please upgrade to avoid potential problems.

Given a MutableRoaringBitmap, you can simply cast it to an ImmutableRoaringBitmap if you wish to have an ImmutableRoaringBitmap. In such a case, ImmutableRoaringBitmap will be effectively backed by a ByteBuffer held in RAM.