Better dev reloading

In development, we currently use shotgun if we want code reloading and thin if we want speed.

There are some problems with shotgun though:
* it's slow - it boots the entire app for every request
* it has a bug where doesn't log in development

mr-sparkles on the other hand, only reloads when files change, so is much faster.

Also this means using unicorn in development, which should be good for dev-prod parity.