Whitelist Environment Agency host

UKBA has now transitioned and so mostly doesn't work in the
side-by-side browser now, but it is used in tests here so needs to
stay in the whitelist for now. EA is the next big transitioning site
so it's useful to have it whitelisted for development, even though
in production we use a full list of hosts from Transition's hosts API.
Update xip.io example in README to use EA

...so that we have a working example for development use.
Merge pull request #5 from alphagov/add_ea_to_whitelist

Add Environment Agency to whitelist