travis: Fix make distcheck failure

travis: Fix make distcheck failure

Since commit f500c49c8e0, the man page and html documentation need to be
generated when building out of the git repository, as both openvpn.8 and
openvpn.8.html will be shipped pregenerated inside the tarball generated
by 'make dist'.

Travis was lacking the python-docutils package, which made the
'make distcheck' build test fail.

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20200720103822.26088-1-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20497.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>