Fix the handling of file names in CucumberKeywords
Fix the handling of file names in CucumberKeywords

The Yaml parser does not accept file names anymore in Symfony 3 (and
this is deprecated in Symfony 2).
File names are now handled in the CucumberKeywords class directly now,
copying the logic available in symfony/yaml 2.x, as we want to continue
accepting a filename.

Behat/Gherkin,stof/Gherkin