Fix #135 by finding metaschemas relative to gem_root

Addresses #135.
It's uncommon to see direct access to `Gem` in lib code. It'd be uglier, but I think the more typical pattern for this would be something like:

~~~ruby
# didn't bother to test this; +/- a few ../s probably =)
resources = File.expand_path('../../../../../resources', __FILE__)
File.join(resources, @metaschema_name)
~~~

That could be cleaned up by just exposing `JSON::Schema.metaschema('draft-04.json')` or similar -- the path would be much closer to the root at that point.