Hotfix for Symfony 2.6 Compatibility

In 2.6, OptionsResolverInterface has changed, `replaceDefaults()` acts differently now (replaces everything) where `setDefaults()` merges the differences 

This is a temporary fix, as `OptionsResolverInterface` has been marked as Deprecated and planned to be removed for Symfony 3.0

It is recommended instead to simply use `OptionsResolver`, but currently this doesn't not coincide with a proper override and breaks the library entirely.