Clean Carrierwave upload directory after every example
Clean Carrierwave upload directory after every example

Previously we were only cleaning this directory once at the end of the
sutie. I'm about to add some examples which need to make assertions
about files and directories within this upload directory and doing so
would've been much harder without this change.

There is probably some performance cost to calling this after every
spec, but I think it's pretty negligible.

alphagov/asset-manager,alphagov/asset-manager,alphagov/asset-manager