Correctly disable the cache in development

Setting a cache_ttl only has an effect on responses that don't include an
Expires header. Content API now issues Expires headers, so this configuration
wasn't doing what it was intended to do.

Now that we have a semantic way of saying "no caching please", let's just use
that.
Merge pull request #386 from alphagov/correctly_disable_cache_in_dev

Correctly disable the cache in development