Add explicit GOV.UK in the design principles title

https://www.google.co.uk/search?q=design+principles gives a first result
with ‘GDS design principles - Gov.UK’ listed.

Obviously Google don’t understand that we’re capped
up since we mean business!
Update Ruby version patchlevel
Update govuk_frontend_toolkit gem
Remove A-Z configuration

This is always true now.
Use the new alpha/beta banner
Remove _conditionals.scss file

The mixin defined in this file isn't used anywhere and shouldn't
ever be used.