.travis.yml: The 'sudo' tag is now deprecated in Travis CI (#400)
.travis.yml: The 'sudo' tag is now deprecated in Travis CI (#400)

[Travis are now recommending removing the __sudo__ tag](https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration).

"_If you currently specify __sudo: false__ in your __.travis.yml__, we recommend removing that configuration_"

Also, do not hard-code __Trusty__ because it reaches its end-of-support this month.
https://wiki.ubuntu.com/Releases
RobotWebTools/rosbridge_suite