Check that the R?Spec::Matchers namespace exists, not just its parent.

Cucumber defines a Spec::Runner module but not a Spec::Matchers module, which causes

    uninitialized constant Spec::Matchers (NameError)

I see this was raised and acknowledged in https://github.com/mbklein/equivalent-xml/pull/5, but not merged. Had a quick check and `defined?(::Foo::Bar)` doesn't cause an exception here (on 1.9.3 at least) so I think this should be good to merge.