Use new NumbersRelativeOff 7.3.1115 compatibility.

VIM versions 7.3.1115 and greater's nu and rnu options interact differently.
See https://github.com/myusuf3/numbers.vim/pull/32\#issuecomment-20896836 or
:h number_relativenumber
Merge pull request #34 from RyanMcG/multiple-version-compatible

All version compatible handling of rnu option