.travis.yml: build documents under bionic

It looks like the xenial tooling doesn't like something in our setup.
We should probably be moving to bionic for everything soon
anyway (libssh aside).

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200203090932.19147-10-alex.bennee@linaro.org>
1 file changed