get_db_prep_value method signature mismatch

In pickledfield.fields, class PickledObjectField, the method:

```
def get_db_prep_value(self, value):
```

needs to have the following signature to be compatible with newer versions of Django:

```
def get_db_prep_value(self, value, connection, prepared = False):
```

and you'll want to add the following at the start of the method:

```
    if prepared:
        return value
```
