Use only the DBM mapping interface.

Not all DBM backends support the same operations.  The only guaranteed API that
works consistently across all "anydbm" supported modules is the mapping
interface.  In particular, the ".get()" method is not implemented by bsddbm,
dumbdbm nor dbm backends.
Merge branch 'master' of git://github.com/etanol/sorl-thumbnail into etanol-master

* 'master' of git://github.com/etanol/sorl-thumbnail:
  Use only the DBM mapping interface.
Merge branch 'etanol-master'

* etanol-master:
  Use only the DBM mapping interface.