travis: Workaround skipping of IPv6 tests.

IPv6 support disabled in TravisCI images but supported by kernel.
So, we could enable it in order to not skip unit tests.
We are not trying to communicate over network with IPv6, so this
should not make any harm.

Related issue: travis-ci/travis-ci#8891

Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>

.travis/linux-prepare.sh

@@ -20,3 +20,8 @@ if [ "$M32" ]; then
     # This will remove the 64-bit libunwind and install 32-bit version.
     sudo apt-get install -y libunwind-dev:i386
 fi
+
+# IPv6 is supported by kernel but disabled in TravisCI images:
+#   https://github.com/travis-ci/travis-ci/issues/8891
+# Enable it to avoid skipping of IPv6 related tests.
+sudo sysctl -w net.ipv6.conf.all.disable_ipv6=0