migrate Travis testing from Trusty to Xenial

[toozej]

Xenial is now finally supported on Travis, so move automated testing from Trusty to Xenial

https://docs.travis-ci.com/user/reference/xenial/ might be useful

things to do:

• [x] dockerfile
• [x] search all variables files for 14.04-specific things
• [x] search package names that are specific for 14.04 vs 16.04
• [x] search for word trusty or 14.04

[toozej]

done