Fix type argument in parser.add_argument for django > 1.8

[miigotu]

argparse's type argument expects a callable, not a string representing a type Fixes Error parsing command thumbnail_cleanup: 'int' is not callable https://docs.python.org/3/library/argparse.html https://docs.python.org/2/library/argparse.html

@SmileyChris can you merge this please?

[lechup]

I've just wanted to create exactly the same PR. You were month faster @miigotu ;)

@SmileyChris :+1: for merging it to master.

[wismill]

+1

[lechup]

@SmileyChris, any chance to have it merged? Do You plan to release 2.4.2?