Upgrade wpebackend-fdo to 1.9.91
This was downgraded in !1097 (merged) because the 1.9.90 release was bad. 1.9.91 is out now with the fixes we needed, and I want to test it.
This was downgraded in !1097 (merged) because the 1.9.90 release was bad. 1.9.91 is out now with the fixes we needed, and I want to test it.