Merge branch 'hotfix/3827'

Close #3827
Fix failing tests due to symfony/yaml autoloading

- phpunit/phpunit was relying on functionality from symfony/yaml that was
  removed in version 2.2.0; pinning symfony/yaml to 2.1.* solves those issues.
Merge branch 'hotfix/yaml-dev-version' into develop

Fixes for symfony/yaml autoloading