Use npm badge for version in README (github release api seems unresponsive too often)

Author: MoOx

README.md

@@ -3,7 +3,7 @@
 [![Travis Build Status](https://img.shields.io/travis/MoOx/postcss-cssnext.svg?label=unix%20build)](https://travis-ci.org/cssnext/postcss-cssnext)
 [![AppVeyor Build Status](https://img.shields.io/appveyor/ci/MoOx/postcss-cssnext.svg?label=windows%20build)](https://ci.appveyor.com/project/MoOx/postcss-cssnext)
 [![Package Quality](http://npm.packagequality.com/shield/postcss-cssnext.svg)](http://packagequality.com/#?package=postcss-cssnext)
-[![Version](https://img.shields.io/github/release/MoOx/postcss-cssnext.svg)](https://github.com/MoOx/postcss-cssnext/blob/master/CHANGELOG.md)
+[![Version](https://img.shields.io/npm/v/postcss-cssnext.svg)](https://github.com/MoOx/postcss-cssnext/blob/master/CHANGELOG.md)
 [![Support on gitter chat](https://img.shields.io/badge/support-gitter%20chat-E40255.svg)](https://gitter.im/MoOx/postcss-cssnext)