Pickle Column Error

If my Model has a Pickle column, then when I run alembic upgrade will complain an error, I found the versions file will be:

```
sa.Column('content', sa.PickleType(pickler=<module 'cPickle' (built-in)>), nullable=True)
```

So I think it's a bug. I can change above as:

```
sa.Column('content', sa.PickleType(), nullable=True)
```

Then the error is fixed.
