Aesthetic fix for current-line gutter highlight

Fix for the new relative number with absolute numbering as of Vim [Patch 7.3.1115](ftp://ftp.vim.org/pub/vim/patches/7.3/7.3.1115). The numbers.vim plugin uses the new relative numbering and the `LineNr` highlight was inconsistent (and vim-gitgutter mirrored this inconsistency).

![lineno](https://f.cloud.github.com/assets/1911028/944122/dab165f4-0296-11e3-83ff-44edf83454f9.png)

`highlight clear LineNr` resolves this inconsistency.