Switch to shields.io for badges in README

Closes #15380
[ci skip]

README.md

@@ -1,8 +1,8 @@
 # [Bootstrap](http://getbootstrap.com)
-[![Bower version](https://badge.fury.io/bo/bootstrap.svg)](http://badge.fury.io/bo/bootstrap)
-[![npm version](https://badge.fury.io/js/bootstrap.svg)](http://badge.fury.io/js/bootstrap)
-[![Build Status](https://secure.travis-ci.org/twbs/bootstrap.svg?branch=master)](https://travis-ci.org/twbs/bootstrap)
-[![devDependency Status](https://david-dm.org/twbs/bootstrap/dev-status.svg)](https://david-dm.org/twbs/bootstrap#info=devDependencies)
+![Bower version](https://img.shields.io/bower/v/bootstrap.svg?style=flat)
+[![npm version](https://img.shields.io/npm/v/bootstrap.svg?style=flat)](https://www.npmjs.com/package/bootstrap)
+[![Build Status](https://img.shields.io/travis/twbs/bootstrap/master.svg?style=flat)](https://travis-ci.org/twbs/bootstrap)
+[![devDependency Status](https://img.shields.io/david/dev/twbs/bootstrap.svg?style=flat)](https://david-dm.org/twbs/bootstrap#info=devDependencies)
 [![Selenium Test Status](https://saucelabs.com/browser-matrix/bootstrap.svg)](https://saucelabs.com/u/bootstrap)
 
 Bootstrap is a sleek, intuitive, and powerful front-end framework for faster and easier web development, created by [Mark Otto](https://twitter.com/mdo) and [Jacob Thornton](https://twitter.com/fat), and maintained by the [core team](https://github.com/orgs/twbs/people) with the massive support and involvement of the community.