Better dev reloading

Twin of https://github.com/alphagov/rummager/pull/138

Try it out with: 
```bash
bundle exec mr-sparkle --force-polling -- -p 3022
```

Though if you want it to reload on changes to ``rabl`` files, you'll need something like:
```bash
bundle exec mr-sparkle --force-polling --pattern "rabl|rb|ru|txt|yml" -- -p 3022
```
I'll add this latter form to https://github.gds/gds/development/blob/master/Procfile#L20