Use SVG badges instead of PNG badges

[![Build Status](https://travis-ci.org/yyx990803/vue.png?branch=master)](https://travis-ci.org/yyx990803/vue) → [![Build Status](https://travis-ci.org/yyx990803/vue.svg?branch=master)](https://travis-ci.org/yyx990803/vue)
[![Coverage Status](https://coveralls.io/repos/yyx990803/vue/badge.png)](https://coveralls.io/r/yyx990803/vue) → [![Coverage Status](https://img.shields.io/coveralls/yyx990803/vue.svg)](https://coveralls.io/r/yyx990803/vue?branch=master)

SVG badges look beautiful on retina displays.