gs-content-rating: Update categories for ESRA rating system
It seems that, at some point in the last 3 years, ESRA has changed their set of ratings categories from +3, +7, +12, +18, +25 to +3, +7, +12, +15, +18
Update our list to match.
Signed-off-by: Philip Withnall withnall@endlessm.com