Bump org.yaml:snakeyaml from 2.5 to 2.6 in /core

Bumps [org.yaml:snakeyaml](https://bitbucket.org/snakeyaml/snakeyaml) from 2.5 to 2.6.
- [Commits](https://bitbucket.org/snakeyaml/snakeyaml/branches/compare/snakeyaml-2.6..snakeyaml-2.5)

---
updated-dependencies:
- dependency-name: org.yaml:snakeyaml
  dependency-version: '2.6'
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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