travis.yml: switch to Ubuntu 14.04 LTS "Trusty Tahr".
travis.yml: switch to Ubuntu 14.04 LTS "Trusty Tahr".

Ubuntu 12.04 LTS "Precise Pangolin" will reach end of life soon.

https://blog.travis-ci.com/2017-04-17-precise-EOL

jwilk/pydiatra,jwilk/pydiatra