Add a blanket 1 hour cache to all responses

Varnish or equivalent can then cache our responses for us.
Re-middleware caching
Don't cache 500s
Idiomatic RSpec
Defer garbage collection for an RSpec boost

Going to idiomatic RSpec slowed the tests down a bit. This commit helps with performance of the specs.
See spec/support/deferred_garbage_collection.rb for link and further details
Merge pull request #14 from alphagov/idiomatic-rspec

Caching middleware / Idiomatic RSpec