example-data-wallet_bdb_1 exited with code 1

After run 'docker-compose up ' , it throws:
```
bdb_1     |     'BigchainDB currently supports {}'.format(backend, BACKENDS.keys()))
bdb_1     | bigchaindb.common.exceptions.ConfigurationError: Backend `mongodb` is not supported. 

BigchainDB currently supports dict_keys(['localmongodb'])
```