travis: Remove usage of 'sudo'

Travis CI reports

  root: deprecated key sudo (The key `sudo` has no effect anymore.)

so let's drop it.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

.travis.yml

-sudo: required
 language: c
 compiler: clang
 os: osx