docs(README): use svg badge

Request Type: docs

How to reproduce: 

Component(s): 

Impact: small

Complexity: small

This issue is related to: https://github.com/angular/angular.js/pull/6610.

**Detailed Description:**

Use svg badge so it looks nicer on retina displays.

**Other Comments:**

Travis just updated their badge to use svg yesterday - https://github.com/travis-ci/travis-ci/issues/630#issuecomment-38054967.