datapath: Support kernel version 4.19.x and 4.20.x

datapath: Support kernel version 4.19.x and 4.20.x

This patch updated acinclude.m4 so that OVS can be compiled on 4.19.x
and 4.20.x kernels.
This patch also updated travis files so that latest kernel versions
are used during travis test builds.

Tested-by: Greg Rose <gvrose8192@gmail.com>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>
Acked-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Yifeng Sun <pkusunyifeng@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>