Fix #135 by finding metaschemas relative to gem_root

Addresses #135.
Nah I'd disagree with that. The other places that refer to Gem::Specification are used to load external gems. Using Gem::Specification to get the gem root seems like a really bad way to get to a relative path. I have a feeling that when you run the test suite, this might access the version you have installed via ruby gems, rather than the checked out version you are testing.

If it helps, I think it might be as simple as `resources = File.expand_path('../../../resources', __dir__)`