maint: relax autoconf requirement to v2.68.

* configure.ac (AC_INIT): 2.68 is new enough, and is the newest
version available by default on Travis CI server.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
maint: install help2man for travis.

* .travis.yml (install): help2man is a build requirement.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>