Update the logo to match front page of nixos.org

Seems like the community was in support of changing it, and someone went and changed it on the homepage, so we might as well stay consistent.

+1 -1
+1 -1
README.md
··· 1 - [<img src="http://nixos.org/logo/nixos-hires.png" width="500px" alt="logo" />](https://nixos.org/nixos) 1 + [<img src="http://nixos.org/logo/nixos-logo-only-hires.png" width="500px" alt="logo" />](https://nixos.org/nixos) 2 2 3 3 [![Build Status](https://travis-ci.org/NixOS/nixpkgs.svg?branch=master)](https://travis-ci.org/NixOS/nixpkgs) 4 4 [![Issue Stats](http://www.issuestats.com/github/nixos/nixpkgs/badge/pr)](http://www.issuestats.com/github/nixos/nixpkgs)