text: Title: Allow ruamel.yaml package version > 0.18 to avoid CVE-2019-20478
  Body: [This issue](https://nvd.nist.gov/vuln/detail/CVE-2019-20478) appear in all the versions for the ruamel.yaml < 0.18.
  this violations is fixed for the version > 0.18.
  
  
  Would be great if greatexpectation tool is adjusted to use ruamel.yaml package version > 0.18 to get rid of CVE error!
  
  Thanks!
  
  