Use SVG badge instead of PNG badge

I updated image format of the Travis CI badge. SVG badges look beautiful on retina displays.

| PNG ver. | SVG ver. |
|---|---|
|[![Build Status](https://travis-ci.org/jonkemp/gulp-useref.png?branch=master)](https://travis-ci.org/jonkemp/gulp-useref)|[![Build Status](https://travis-ci.org/jonkemp/gulp-useref.svg?branch=master)](https://travis-ci.org/jonkemp/gulp-useref)|