travis: Switch from Docker Hub to quay.io

As it's currently impossible for us to create new automated
builds on Docker Hub (see [1]), and quay.io doesn't suffer
from the same problem while still having all the feature we
need, switch to the latter.

[1] https://github.com/docker/hub-feedback/issues/1676Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

.travis.yml

@@ -46,7 +46,7 @@ script:
       -e VIR_TEST_DEBUG="$VIR_TEST_DEBUG"
       -e MINGW="$MINGW"
       -e DISTCHECK_CONFIGURE_FLAGS="$DISTCHECK_CONFIGURE_FLAGS"
-      "libvirt/buildenv-$IMAGE"
+      "quay.io/libvirt/buildenv-$IMAGE:master"
       /bin/sh -xc "$DOCKER_CMD"
 
 git: