travis: switch to sid

The upcoming Qemu 3.0 build is built using an Ubuntu Bionic (18.04 LTS) PPA as
launchpad does not support building packages for Debian directly.

The dependencies were all too new to use a Ubuntu Cosmic Qemu under
Stretch (Debian stable), but with Bionic/sid they mostly all match.

Signed-off-by: Joel Stanley <joel@jms.id.au>

.travis.yml

@@ -35,7 +35,7 @@ cache:
 services:
   - docker
 script:
-  - docker create -v ${TRAVIS_BUILD_DIR}:/travis --name travis debian
-  - docker run --env ARCH=${ARCH} --env LD=${LD} --rm --volumes-from travis debian /bin/bash -c 'cd /travis && ./debian-setup.sh && ./driver.sh && ccache -s'
+  - docker create -v ${TRAVIS_BUILD_DIR}:/travis --name travis debian:sid
+  - docker run --env ARCH=${ARCH} --env LD=${LD} --rm --volumes-from travis debian:sid /bin/bash -c 'cd /travis && ./debian-setup.sh && ./driver.sh && ccache -s'
 after_script:
   - sleep 1