I believe the fix is to change the set of engines MultiJson#default_engine searches from
%w(yajl json active_support json/pure)
%w(yajl json_gem active_support json/pure)
Alternatively, rename lib/multi_json/engines/json_gem.rb to lib/multi_json/engines/json.rb.
I just hit that with JRuby on Gentoo, I'm actually trying to make a slightly more complex but "fuller" solution…
I fixed it in my “fork”, by using a map between libraries and symbols rather than mangling the symbols.
This issue should be fixed in master, I am closing the issue for now but please reopen it if you still experience issues.