build: Simplify Travis configuration

build: Simplify Travis configuration

* Remove redundant 'group: edge'. Use the default (stable) instead.
* Use simplified syntax for apt packages. On sudo-enabled VMs, like we
  use now, this translates to the same command. But has the benefit
  of being compatible with the fast non-sudo containers, which we
  want to use in the future. But, for that, djvulibre-bin needs to
  be approved first.
  <https://github.com/travis-ci/apt-package-whitelist/issues/4036>

Bug: T75176
Change-Id: I3b42763ef3f8f08eec08a9008e5cf1e161bb1dff