update workflow to include pip-compile

[jason-upchurch]

Summary

pip-compile will let us pin all the dependencies for a deterministic build.

• Move requirements.txt to requirements.in and use pip-compile to generate new requirements.txt.
• Move requirements-dev.txt to requirements-dev.in and use pip-compile to generate new requirements-dev.txt.
• start with a clean environment, e.g., api-env-prod so that "extra" libraries are not included in the committed requirements.txt and requirements-dev.txt
• updated README.md with new workflow
• update config.yml to include correct pip install -r <dependency file> (e.g., remove pip install -r requirements-ci.txt