Rather than raise on an empty log, output a warning.

Just one log needs to be out of whack and this will keep raising
a ton of errors.
Merge branch 'master' into mm-dont-raise-on-empty-logs
Remove jruby-head.
Use latest JRuby.
Merge pull request #13 from travis-ci/mm-dont-raise-on-empty-logs

Don't raise error on empty logs