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

If another module (e.g. Cucumber) defines a Spec module but not a Spec::Matchers module, this causes

    uninitialized constant Spec::Matchers (NameError)
Include rspec/matchers to ensure that Rspec::Matchers is defined
Merge pull request #14 from elandesign/master

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