use svg instead of png

changed travis, fury and npm badges to use the svg badge instead of the png badge

| PNG | SVG |
|:---:|:---:|
| ![NPM version](https://badge.fury.io/js/verb.png) | ![fury.io](https://badge.fury.io/js/verb.svg) |
| ![NPM](https://nodei.co/npm/verb-readme-badges.png) | [![NPM](https://nodei.co/npm/verb-readme-badges.svg)](https://nodei.co/npm/verb-readme-badges.svg) |
| ![Travis CI](https://travis-ci.org/assemble/assemble-middleware-sitemap.png) | ![Travis CI](https://travis-ci.org/assemble/assemble-middleware-sitemap.svg) |