DEV: upgrade json gem and add explicit dependency

json is shipped out of sync with Ruby. Even though we use OJ for many things
we still use the json gem sometimes, this ensures we use the latest

b8b29e79ad/config/initializers/100-oj.rb (L9-L9)

Gemfile

@@ -28,6 +28,8 @@ else
   gem 'sprockets-rails'
 end
 
+gem 'json'
+
 # TODO: At the moment Discourse does not work with Sprockets 4, we would need to correct internals
 # This is a desired upgrade we should get to.
 gem 'sprockets', '3.7.2'

Gemfile.lock

@@ -157,6 +157,7 @@ GEM
       rails-dom-testing (>= 1, < 3)
       railties (>= 4.2.0)
       thor (>= 0.14, < 2.0)
+    json (2.3.0)
     jwt (2.2.1)
     kgio (2.11.3)
     libv8 (7.3.492.27.1)
@@ -467,6 +468,7 @@ DEPENDENCIES
   highline (~> 1.7.0)
   htmlentities
   http_accept_language
+  json
   listen
   lograge
   logstash-event