update riak_test_lager_backend for new msg tuple

The `riak_test_lager_backend` supports multiple versions of lager including lager 1.2.1/2.0.0. When support was added for the latter, the release was still a moving target (master was used). It seems the message structure has changed a bit.

This fixes this 1.4pre1 failure: http://giddyup.basho.com/#/projects/riak/scorecards/32/32-368-verify_busy_dist_port-centos-5-64/11327/artifacts/13721