circe-yaml 0.8.0

This release removes the unnecessary circe-parser dependency and updates the SnakeYAML version from 1.19 to 1.20. It also changes one implicit class into a non-value class because of issues on Scala 2.10, and adds support for "yes", "no", "on", and "off" as boolean values (thanks to Amir M. Saeid in #44).