travis: Use docker container to gain Ubuntu 16.04 build environment.
travis: Use docker container to gain Ubuntu 16.04 build environment.

twoporeguys/librpc,twoporeguys/librpc,twoporeguys/librpc,twoporeguys/librpc,twoporeguys/librpc,twoporeguys/librpc,twoporeguys/librpc,twoporeguys/librpc