Ensure multi_json uses json gem

This can be updated later to swap to Oj, but for
now we need to explicitly swap it away from the :oj
option it will pick up automatically.

Gemfile

@@ -501,3 +501,4 @@ gem 'valid_email', '~> 0.1'
 gem 'json', '~> 2.3.0'
 gem 'json-schema', '~> 2.8.0'
 gem 'oj', '~> 3.10.6'
+gem 'multi_json', '~> 1.14.1'

Gemfile.lock

@@ -1313,6 +1313,7 @@ DEPENDENCIES
   mimemagic (~> 0.3.2)
   mini_magick
   minitest (~> 5.11.0)
+  multi_json (~> 1.14.1)
   nakayoshi_fork (~> 0.0.4)
   net-ldap
   net-ntp

config/initializers/multi_json.rb

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