rewrite rules: add missing last,redirect to the Gravatar 404s
rewrite rules: add missing last,redirect to the Gravatar 404s

Without [last,redirect], it's possible that mod_rewrite would continue
executing for a bit.

libravatar/libravatar,libravatar/libravatar,libravatar/libravatar,libravatar/libravatar,libravatar/libravatar,libravatar/libravatar,libravatar/libravatar