Use svg for build status

Hi!

First of all, thanks for an awesome framework. I have used it for many projects.

Second of all, sorry for the very minor patch. But I just get an itch when I see those pngs on a retina display, knowing that travis offers svg :)

So if you want to use this instead, that makes me happy. Otherwise, no biggie. Keep up the good work, and have a great day!