travis-ci update ubuntu image, see https://blog.travis-ci.com/2017-06-21-trusty-updates-2017-Q2-launch
travis-ci update ubuntu image, see https://blog.travis-ci.com/2017-06-21-trusty-updates-2017-Q2-launch

naiba/stacker,naiba/stacker,Maxlab/stacker,Maxlab/stacker