Increase healthCheckExpire to 20s
Increase healthCheckExpire to 20s

This is failing intermittently on Travis CI. I think the only thing we can
do is increase the wait for the health check to expire :(

mikepea/cdn-acceptance-tests,mikepea/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,mikepea/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,mikepea/cdn-acceptance-tests