Fix issue with MultiJson feature detection.
Fix issue with MultiJson feature detection.

You can't reliably detect whether `MultiJson` responds to `load`
since Kernel defines a `load` method, so use `adapter` as a proxy.

This should resolve #39.

iainbeeston/json-schema,ganeshkumar/json-schema,RST-J/json-schema,openc/json-schema,openc/json-schema,ruby-json-schema/json-schema