FIX #46: Fallback to index.js if no main attribute in package.json

Hey, @lordhoto and I think this case is covered above by your own fix, no? on line 123 (after trying dir/index) you're adding .js and trying again. Maybe you should try deleting this case and rerun the tests, if they all pass then this would be ready to land.

Thanks again, the other fixes all look good.