docs(README): use svg instead of png to get better image quality

Closes #2629

Author: PeterDaveHello

README.md

@@ -2,8 +2,8 @@
 
 ***
 
-[![Build Status](https://secure.travis-ci.org/angular-ui/bootstrap.png)](http://travis-ci.org/angular-ui/bootstrap)
-[![devDependency Status](https://david-dm.org/angular-ui/bootstrap/dev-status.png?branch=master)](https://david-dm.org/angular-ui/bootstrap#info=devDependencies)
+[![Build Status](https://secure.travis-ci.org/angular-ui/bootstrap.svg)](http://travis-ci.org/angular-ui/bootstrap)
+[![devDependency Status](https://david-dm.org/angular-ui/bootstrap/dev-status.svg?branch=master)](https://david-dm.org/angular-ui/bootstrap#info=devDependencies)
 
 ## Demo