TF1.15 compatibility proposal

[howard-tran]

I cloned sparkflow and run with my TF1.15. The runtime log multiple errors which relate to sparkflow So i make some changes toward sparkflow on master branch and all the issue is gone.

I also change from using tf.get_collection(tf.GraphKeys.LOSSES)[0] to tf.get_collection(tf.GraphKeys.GLOBAL_VARIABLES) due to the fact that my model is written in old traditional way using tf.nn instead of tf.layers