Fix import for `virtual` from the SQLAlchemy backend [GH-127]

The SQLAlchemy backend was trying to import `virtual` instead of
`transport.virtual`.

kombu/transport/sqlalchemy/__init__.py

@@ -7,7 +7,7 @@ from sqlalchemy import create_engine
 from sqlalchemy.exc import OperationalError
 from sqlalchemy.orm import sessionmaker
 
-from kombu import virtual
+from kombu.transport import virtual
 from kombu.exceptions import StdChannelError
 
 from .models import Queue, Message, metadata