Fixes #860

 * Relax version requirements for `symfony/yaml`.
Don't use the Symfony Yaml 2.2.0 PEAR autoloader.

See: https://github.com/sebastianbergmann/phpunit/issues/846#issuecomment-14925356
Merge pull request #862 from whatthejeff/yaml_version_juggling
Use Use Symfony YAML PEAR autoloader when available
Merge branch '3.7'