Remove misleading in_preview variable.

govuk_platform is set to production everywhere, so this doesn't behave
as expected.
Set sync=true for production.log

Ensures log writes are flushed out to the OS, resulting in entries
getting into Kibana sooner.

Also tidied up the logging config.
Log requests in json format.

So that we can pull them into Kibana with lots of detail.
Merge pull request #175 from alphagov/json_logging

Add json logging.