Readme: Use an SVG badge.

Using the up-to-date badge design.

Note: I could also use [`https://api.travis-ci.org/wycats/handlebars.js.svg?branch=master`](https://api.travis-ci.org/wycats/handlebars.js.svg?branch=master), but its graphical design is slightly outdated.

Full disclosure: I am the maintainer of the Shields.io project.