Travis: Update to use docker with Bionic target
Travis: Update to use docker with Bionic target

This changes the TravisCI behavior to use docker with a Bionic target.
Provides consistency with gitian target.

Signed-off-by: cevap <34c6fceca75e456f25e7e99531e2425c6c1de443@i2pmail.org>

ionomy/ion,ionomy/ion,cevap/ion,cevap/ion,cevap/ion,cevap/ion,ionomy/ion,ionomy/ion,cevap/ion,ionomy/ion,ionomy/ion,cevap/ion