The ability to pass file names was deprecated

From Yaml::parse: "The ability to pass file names to Yaml::parse() was deprecated in 2.7 and will be removed in 3.0"
there is no reason to bump this requirement: the YAML component supports passing a YAML string since its first release.
and this makes it much harder to use phpspec in a Symfony project (if you install it with composer, it requires you to use Symfony 2.7 in your project)