Use YAML schema for Treeherder messages

[Dave Hunt]

We're currently using a schema converted from YAML to JSON, which means there's an additional steps whenever updating it from the master copy. We should use the unmodified YAML version from the treeherder repository instead.

• Current JSON schema
• Official YAML schema

We could convert the YAML to JSON using something like SnakeYAML, or even better, com.fasterxml.jackson.dataformat.yaml.YAMLFactory.

[Dave Hunt]

Using YAMLFactory from jackson-dataformats-text is likely blocked on #4.