hydra.sql: Make isChannel a not null column.

We have this set in upgrade-42.sql, so it's better to stay consistent with the basic SQL file to avoid problems with new Hydra installations.

Signed-off-by: aszlig <aszlig@redmoonstudios.org> Reported-by: Eelco Dolstra <eelco.dolstra@logicblox.com>

Created by  aszlig  on October 14, 2015
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