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/ad484a92d290c2360b960b1a1d446a761f8f65f9/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. :heart_eyes_cat: