Remove travis sudo option to help switch to xenial

According to
https://blog.travis-ci.com/2019-04-15-xenial-default-build-environment,
xenial is supposed to be the default linux build environment, using
vms. However, our jobs are still using trusty.

I think this might be related to the sudo option, which historically
made sure you were running on a vm and not in a
container. Container-based builds are deprecated as of
https://changelog.travis-ci.com/the-container-based-build-environment-is-fully-deprecated-84517,
so removing the sudo option to see if that helps force the switch to xenial.

Signed-off-by: Mark Stemm <mark.stemm@gmail.com>

.travis.yml

@@ -20,7 +20,6 @@ compiler: gcc
 env:
     - BUILD_TYPE=debug
     - BUILD_TYPE=release
-sudo: required
 services:
     - docker
 before_install: