travis: Drop Ubuntu 16.04 build

This will make further changes easier; all coverage
lost due to this will be reintroduced later on.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

.travis.yml

@@ -13,12 +13,6 @@ matrix:
         - IMAGE=ubuntu:18.04
         - DISTCHECK_CONFIGURE_FLAGS="--with-init-script=systemd"
         - DOCKER_CMD="$LINUX_CMD"
-    - services:
-        - docker
-      env:
-        - IMAGE=ubuntu:16.04
-        - DISTCHECK_CONFIGURE_FLAGS="--with-init-script=upstart"
-        - DOCKER_CMD="$LINUX_CMD"
     - compiler: clang
       language: c
       os: osx