Change of Keras API

[Vopaaz]

When I run pix2pix-keras.ipynb, in the fifth block

if K._BACKEND == 'theano':

raises

AttributeError: module 'keras.backend' has no attribute '_BACKEND'

In the current Keras version (2.2.5), the proper API has changed to K.backend().
Would you please fix that, or should I draft a PR?

Besides, consider that the repo is created 2-3 years ago, there may be a lot of such API change and will be troublesome to fix them all. Would it be helpful to add a requirements.txt stating all the proper version of used packages, to avoiding such problems?

Thanks!

[tjwei]

Thanks, I will modernize the code soon, and move old code to a legacy folder and hopefully make a requirements.txt for them.

[Vopaaz]

Much thanks!