travis: switch to Ubuntu 16.04 LTS “Xenial Xerus” by default.
travis: switch to Ubuntu 16.04 LTS “Xenial Xerus” by default.

Ubuntu 14.04 LTS “Trusty Tahr” has reached end of life.

https://blog.ubuntu.com/2019/02/05/ubuntu-14-04-trusty-tahr

jwilk/mwic,jwilk/mwic