Add check for flash fallback v3_v4 upgrade method

If you don't check for the existence of the property, the upgrade results in an error (Module collective.quickupload.upgrades.upgrades, line 25, in v3_v4 - Module OFS.PropertyManager, line 184, in _setProperty - BadRequest: Invalid or duplicate property id)
Merge pull request #45 from spereverde/master

Add check for flash fallback v3_v4 upgrade method