memcached tests fail only on Travis randomly

See:  https://travis-ci.org/appneta/oboe-ruby/jobs/26894870

If I re-run the test, I get a different set of failures and sometimes it passes.  The number of trace events always varies between 4 and 5.  Since I can't reproduce this locally, we'll likely to do some test commits with debug output on Travis.  Or do something else smart people do.