Switch to SVG badges.

Also use Markdown syntax for the AppVeyor badge and use the master branch.

An example of how it will look in grunt-contrib-less:

```md
# grunt-contrib-less v0.12.0 [![Build Status: Linux](https://travis-ci.org/gruntjs/grunt-contrib-less.svg?branch=master)](https://travis-ci.org/gruntjs/grunt-contrib-less) [![Build Status: Windows](https://ci.appveyor.com/api/projects/status/e3aa4d07xe4w4u05/branch/master)](https://ci.appveyor.com/project/gruntjs/grunt-contrib-less/branch/master)
```

># grunt-contrib-less v0.12.0 [![Build Status: Linux](https://travis-ci.org/gruntjs/grunt-contrib-less.svg?branch=master)](https://travis-ci.org/gruntjs/grunt-contrib-less) [![Build Status: Windows](https://ci.appveyor.com/api/projects/status/e3aa4d07xe4w4u05/branch/master)](https://ci.appveyor.com/project/gruntjs/grunt-contrib-less/branch/master)


/CC @vladikoff