Update carrierwave version

This version includes this fix:
https://github.com/carrierwaveuploader/carrierwave/pull/1517

There's no recent release so I've just pinned to the latest commit on master. Worryingly the carrierwave build is failing but the fix was only merged a few days ago...

@stevegraham @dan-palmer 
should be 
```ruby
gem 'carrierwave', github: 'carrierwaveuploader/carrierwave', ref: '56873b07105396e0f1cfdd5e38be930dc3dcd362'
```

Also what does this fix and where is the regression test?