1.8 Won't Launch

Put the summary of the bug you're having here

What I expected to happen was...: The launcher to take me to Minecraft 1.8

What actually happened was...: Launcher opened, took me to the 'Mojang' loading screen, then crashed. It has done this with 1.7.4 - 1.8

Steps to Reproduce:
1. Launch Minecraft 1.8
2. Push 'Play'
3. Wait for it to crash...