Cleanup output from maven during travis build by using quiet mode

This just makes Maven a lot let verbose. Compare [this](https://travis-ci.org/luuse/opentsdb/jobs/29584279) as it is now with how it looks [after](https://travis-ci.org/luuse/opentsdb/jobs/29599268) this change. With this change it will only output errors and a summary of the executed tests (and more details on errors). Sorry I missed this in the initial PR...