Remove references to Skyscape preview

Our Skyscape preview environment no longer exists.
Remove notskyscape tag

This isn't used by any tests, so it can be removed.
Remove duplicated test

This test passes the same params as test:preview, which makes it
redundant.
Update Rakefile descriptions

The descriptions all had an emphasis on the local network which
no longer reflects the majority of tests we write.
Update README
Remove notproduction tag

No tests use this, because all of our tests work in production.

Aren't we angelic or something?
Merge pull request #105 from alphagov/remove-skyscape-preview

Remove references to Skyscape preview