Specify tensorflow version 1.15.0 to prevent some errors

[colus001]

1. Error while config

AttributeError: module 'tensorflow' has no attribute 'space_to_depth'

2. Error while running test_deep_league.py

Using TensorFlow backend.
Traceback (most recent call last):
File "test_deep_league.py", line 152, in <module>
    sess = K.get_session()  # TODO: Remove dependence on Tensorflow session.
File "/Users/alma/opt/miniconda3/envs/DeepLeague/lib/python3.7/site-packages/keras/backend/tensorflow_backend.py", line 379, in get_session
    '`get_session` is not available '
RuntimeError: `get_session` is not available when using TensorFlow 2.0.