Add 'Compliant' support for IE11 as it's released

Change message to just say IE7+ because it's an inevitability
that new ones will get added.
Move 'Compliant' support from Safari 5 to 7

Safari 5 is from 2010.
Add 'Compliant' support for iOS 7

It's got a decent enough proportion of users on it.
Typo: Blackberry -> BlackBerry
Merge pull request #279 from alphagov/ios-7

Update browser support guidelines