fix php version issue from ErrorResponsePlugin

[sbutnariu]

The method used for verifying that a class implementes an interface (is_subclass_of) did not worked with interfaces before php 5.3.7

[mtdowling]

This is pretty heavy handed for simple validation. I wonder if the validation should just be removed instead?

[sbutnariu]

If the validation is removed, users can use classes that don't implement static method 'fromCommand' , and an error will issue when calling this method at line 71

[mtdowling]

Yeah, and either way the end-user runs into an error that they really can't recover from.

I think a better option would be to use is_callable() and not enforce the interface.