Switch to K.image_data_format

[evictor]

I noticed that keras.backend.image_dim_ordering has been replaced by image_data_format (same package). I was going to submit a PR for py-agender to use this new symbol, but I'm not aware if this will cause backwards compatibility issues and how you might like to handle that.

Please advise. For now I am monkey patching like so in my code (drop-in replacement):

# Patch for PyAgender usage of an old API
from keras import backend as K
K.image_dim_ordering = K.image_data_format