Delete Photoshop images :no_good:

Why (the fuck) should I have to buy a $699 piece of proprietary shitware (that writes proprietary files) to hack on an open source project?

I’d propose that we delete `shields.psd` and stick with SVG and PNG images.

I’d also propose that we remove [Illustrator noise](https://github.com/olivierlacan/shields/blob/bdef94c95a8f4ec22a60f867d81b93d38d7579c7/travis/travis_passing.svg#L2) from the SVGs and make sure they can be converted to PNGs using open source tools like `svg2png` or `convert`.

Branch name courtesy of [@nrrrdcore](http://www.youtube.com/watch?v=e7-ZnHMDZto).