Travis CI: The sudo tag is deprecated

[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_"

.travis.yml

 language: cpp
 cache: ccache
-sudo: required
 dist: trusty
 services:
   - docker