Use a snapshot of Open Babel master

A snapshot of Open Babel master (d6c04d1), this contains several bug
fixes and updates required in Avogadro and MongoChem.

Change-Id: I25754907d292f8933ea4a98f43e912a04de717e9
Merge branch 'openbabel-master'

Change-Id: I62d27995ddb3c559d9744e601667682864005dee