docs(readme): update downloads badge to point to graph of downloads

README.md

@@ -1,5 +1,5 @@
 # Karma
-[![js-standard-style](https://img.shields.io/badge/code%20style-standard-brightgreen.svg?style=flat-square)](https://github.com/karma-runner/karma) [![npm version](https://img.shields.io/npm/v/karma.svg?style=flat-square)](https://www.npmjs.com/package/karma) [![npm downloads](https://img.shields.io/npm/dm/karma.svg?style=flat-square)](https://www.npmjs.com/package/karma)
+[![js-standard-style](https://img.shields.io/badge/code%20style-standard-brightgreen.svg?style=flat-square)](https://github.com/karma-runner/karma) [![npm version](https://img.shields.io/npm/v/karma.svg?style=flat-square)](https://www.npmjs.com/package/karma) [![npm downloads](https://img.shields.io/npm/dm/karma.svg?style=flat-square)](https://npmcharts.com/compare/karma?minimal=true)
 
 [![Build Status](https://img.shields.io/travis/karma-runner/karma/master.svg?style=flat-square)](https://travis-ci.org/karma-runner/karma) [![Build Status](https://img.shields.io/appveyor/ci/dignifiedquire/karma/master.svg?style=flat-square)](https://ci.appveyor.com/project/dignifiedquire/karma) [![Code Climate](https://img.shields.io/codeclimate/github/karma-runner/karma.svg?style=flat-square)](https://codeclimate.com/github/karma-runner/karma) [![PRs Welcome](https://img.shields.io/badge/prs-welcome-brightgreen.svg?style=flat-square)](http://makeapullrequest.com) [![Dependency Status](https://img.shields.io/david/karma-runner/karma.svg?style=flat-square)](https://david-dm.org/karma-runner/karma) [![devDependency Status](https://img.shields.io/david/dev/karma-runner/karma.svg?style=flat-square)](https://david-dm.org/karma-runner/karma#info=devDependencies)