Default multi_json to Oj

Changes the default JSON processor for multi_json to Oj. This
mostly applies to external gems that use multi_json.

Changelog: changed

config/initializers/multi_json.rb

@@ -2,4 +2,4 @@
 
 # Explicitly set the JSON adapter used by MultiJson
 # Currently we want this to default to the existing json gem
-MultiJson.use(:json_gem)
+MultiJson.use(:oj)