Remove no longer existing david-dm badge

Author: gustavohenke

README.md

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 # Toposort
 [![Build Status](https://img.shields.io/travis/gustavohenke/toposort/master.svg)](https://travis-ci.org/gustavohenke/toposort)
-[![Dependency Status](https://img.shields.io/david/dev/gustavohenke/toposort.svg)](https://david-dm.org/gustavohenke/toposort?type=dev)
 
 __Sorting directed acyclic graphs, for Node.js, io.js and the browser__
 _This was originally done by Marcel Klehr. [Why not checkout his original repo?](https://github.com/marcelklehr/toposort)_