pyyaml vulnerability

https://nvd.nist.gov/vuln/detail/CVE-2017-18342

Details: https://github.com/yaml/pyyaml/issues/243#issuecomment-451540701

The suggested version 4.1 is a prerelease so not a good option currently.

According to the details link above using `yaml.safe_load` instead of `yaml.load` fixes this issue.