This change added a version flag to chunk header
``ChunkHeader()`` no longer works! You now have to do
``ChunkHeader(version = '0.6')`` or ``ChunkHeader(version = '0.7')``
This fixes 0.6 chunk header packing.
It has a suddle difference to 0.7 and it is now packed correctly.
Also unpacking 0.6.5 chunk headers is fixed by reading the correct size.
