Sort files in DirectoryIterator because os.walk orders them arbitrarily

[JakeBecker]

The filenames yielded by the DirectoryIterator are in arbitrary order because it uses os.walk (which uses os.listdir under the hood. The docs for os.listdir mention that the filenames are ordered arbitrarily: https://docs.python.org/3/library/os.html#os.listdir . On Mac OSX, the filenames seem to be sorted lexicographically, but on Ubuntu 17.04 with ext4 filesystem, they seem to be given in an arbitary, non-lexicographic order.

The Keras docs give an example of combining images with masks using flow_from_directory, which requires that the files be sorted: https://keras.io/preprocessing/image/ . This fix is important for that to work on all systems.

Fixes #6418

[fchollet]

LGTM