Don't serve content on /

Previously this showed the detault rails index page.

I decided against redirecting / to /business-support-schemes, as
/ should only be seen during dev, and redirecting that to something
meaningful may be confusing.

I think serving a 404 will a) make it clear / isn't a valid route
and b) means no more rails default
Merge pull request #24 from alphagov/no-rails-index-page

Don't serve content on /