use yaml.safe_load instead of yaml.load

[chriselion]

This gets rid of these warnings

/.../ml-agents/ml-agents/mlagents/trainers/learn.py:157: YAMLLoadWarning: calling yaml.load() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details.
trainer_config = yaml.load(data_file)

when running mlagents-learn. It's also just a generally good idea; see the link for more info.

[chriselion]

Tests failing until #2125 is merged in