TRAVIS: allow failure for ubuntu xenial builds
TRAVIS: allow failure for ubuntu xenial builds

Ubuntu Xenial seems to be a bit unstable still and we depend on successful
Rapicorn Xenial package builds.

Signed-off-by: Tim Janik <6ec6ff229c4e1c5bbd5933ee92e6c5832ff023bb@gnu.org>

tim-janik/beast,tim-janik/beast,swesterfeld/beast,swesterfeld/beast,GNOME/beast,GNOME/beast,swesterfeld/beast,tim-janik/beast,swesterfeld/beast,GNOME/beast,GNOME/beast,tim-janik/beast,GNOME/beast,GNOME/beast,swesterfeld/beast,swesterfeld/beast,tim-janik/beast,tim-janik/beast,GNOME/beast,swesterfeld/beast,tim-janik/beast