A small change which wlil keep this module working with the future Moose

There was a useless call to ->_meta_instance in the Constructor trait. The _meta_instance method will be going away in the next Moose version.

This change works with both current _and_ future Moose.

For some reason github is telling me there are 11 changes to pull, but AFAICT it should only be one. If necessary, I can recreate my fork and reapply the patch.