Make Mojo::IOLoop recommended instead of required.

Mojolicious requires Perl 5.10, while Promises with AnyEvent works on Perl 5.8.

This PR allows the test suite to run on Perl 5.8.9 when Mojo::IOLoop is not installed.  Also needs the META file to be updated.

Would appreciate a new release soon as this is all that is blocking my async Elasticsearch release.

thanks stevan