Need to update `install_requires` in setup.py

[keisen]

Version 0.5.0 also introduces keras.backend.identity via this PR

But this isn't introduced in Keras until version 2.0.4.

So the current install_requires=['keras>=2.0', 'six', 'scikit-image', 'matplotlib', 'h5py'] needs to be updated to install_requires=['keras>=2.0.4', 'six', 'scikit-image', 'matplotlib', 'h5py']

Originally posted by @thommiano in #198 (comment)