[PATCH] manual/install.texi: Bump required version of texinfo to 4.7

[Joseph Myers]

On Thu, 11 Dec 2014, Will Newton wrote:

> It seems we require texinfo 4.7 for the --plaintext option, so
> document that.

The version is checked in configure.ac, so that needs updating as well to 
change the requirement.

-- 
Joseph S. Myers
joseph@codesourcery.com