Remove the Travis build status image from the README

This information isn't useful anymore since we disabled all Travis CI
builds in 44917c46b1.

README.md

@@ -1,6 +1,5 @@
 [<img src="http://nixos.org/logo/nixos-hires.png" width="500px" alt="logo" />](https://nixos.org/nixos)
 
-[![Build Status](https://travis-ci.org/NixOS/nixpkgs.svg?branch=master)](https://travis-ci.org/NixOS/nixpkgs)
 [![Code Triagers Badge](https://www.codetriage.com/nixos/nixpkgs/badges/users.svg)](https://www.codetriage.com/nixos/nixpkgs)
 
 Nixpkgs is a collection of packages for the [Nix](https://nixos.org/nix/) package