workaround set_iarc_storefront_data in rare case of missing version (bug 967638)

http://sentry.mktmon.services.phx1.mozilla.com/mkt/marketplacefirefoxcom/group/3779/ hasn't been seen for 17 days, and a lot of the version code has changed. But let's add a fallback Just In Case ```current_version``` is ever ```None```.

Also move content ratings into their own test class.

@robhudson r?