removed vendor prefix from `keyframes pulsate`

The header background animation on Tachyons.io didn't render on FireFox, only a white background (which hid the white "view on github" button). Removing the vendor prefix from _animate_pulse.scss allows Autoprefixer to do its job and render as expected on FF.

ps - this is my first pr ever. Hope I am correct in creating a new branch for this quick fix.

![](http://www.rolsocial.com/blog/wp-content/uploads/GIF-Animated-Dancing-Cat.gif)