Bump gunicorn timeout for write.backdrop

We've been having issues with pushing bigger amounts of data that causes
write.backdrop to get restarted by gunicorn because of large payloads.
In order to mitigate this we need to allow the configuration of timeouts
on gunicorn_apps and bump it specifically for write.backdrop
Merge pull request #263 from alphagov/bump-gunicorn-timeout

Bump gunicorn timeout for write.backdrop