Use bionic in Travis CI (#4660)
Use bionic in Travis CI (#4660)

Ubuntu bionic image is available now.

To switch to bionic, change travis.yml according to follows.
* https://docs.travis-ci.com/user/reference/bionic/
* https://docs.travis-ci.com/user/installing-dependencies/
* https://config.travis-ci.com/ref/job/addons/apt
kubernetes/dashboard,kubernetes/dashboard,kubernetes/dashboard,floreks/dashboard,kubernetes/dashboard,floreks/dashboard,kubernetes/dashboard,floreks/dashboard,floreks/dashboard,floreks/dashboard