travis: run "make distcheck" instead of just "make check"
Running "make distcheck" includes the "make check", and "make dist"
targets. It ensures that we have CLEANFILES and uninstall rules setup
correctly, as well as validating VPATH builds succeed.
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>