fix migrations for python 3

[multigl]

django migration file 0002_auto_20151202_1551.py introduced bytestring literals on the choices kwarg for djangocms_file.models.File.target field

as the model itself was not changed, running with djangocms_file in a python 3.x environment complains that migrations are missing.

This can be fixed by removing the b'' literals in 0002_auto_20151202_1551.py or with a new migration