Documentation: Update required Git version to 1.7.2

Our 'git-gitlab-push' script requires Git 1.7.2 or greater to use the
'git -c <name>=<value> ...' configuration option.  Since 1.7.2 came out
in 2010 it is reasonable to expect developers to have it in 2015.

Documentation/dev/git/README.md

@@ -8,7 +8,7 @@ Main Tasks
 ----------
 
 * [Install Git](http://public.kitware.com/Wiki/Git/Download) -
-  Git 1.6.6 or greater is preferred (required for development)
+  Git 1.7.2 or greater is preferred (required for development)
 
 * [Download VTK](download.md) - Users start here