Running local saber on Amazon Linux causes errors

Running a local execution workflow on an EC2 instance with the Amazon Linux AMI results in critical errors.

Example workflows produce the following error:

```
[2019-11-07 21:57:11,159] {docker_operator.py:169} INFO - Starting docker container from image aplbrain/boss-access:local
[2019-11-07 21:57:11,213] {models.py:1736} ERROR - create_container() got an unexpected keyword argument 'cpu_shares'
Traceback (most recent call last):
File "/usr/local/lib/python3.6/site-packages/airflow/models.py", line 1633, in _run_raw_task
result = task_copy.execute(context=context)
File "/conduit/utils/saber_docker_operator.py", line 30, in execute
super().execute(*args, **kwargs)
File "/usr/local/lib/python3.6/site-packages/airflow/operators/docker_operator.py", line 210, in execute
working_dir=self.working_dir
TypeError: create_container() got an unexpected keyword argument 'cpu_shares'
```