Minimum Texinfo version for creating the documentation

[Rik]

All,

Currently the minimum Texinfo version required for creating the Octave
manual is something ancient like 4.13.  Recent changesets use features from
5.0 (Feb. 2013) and 6.1 (Feb. 2016).  These really improve the appearance
of the manual, but aren't necessarily available on ancient systems.  Given
that Octave creates the manual and distributes it in the tarball, is it
okay to bump up the required Texinfo to 6.1?  Developers on older systems
can use '--disable-docs' to concentrate on just the Octave core, or they
can compile and install newer versions of Texinfo.

--Rik