Hardening AWS EC2 AMI image

[pmeenan]

There's always the chance that an update will break something but it hasn't been a problem in practice and I should notice really quickly since it will take out most of WebPageTest's testing.

The bigger impact will be that you will always have the latest browsers and they will update automatically whenever the browser is released so you may need to check browser versions if you investigate a regression to see if it might have been from a browser change.