Hardwire multi_json backend

The `multi_json` gem dynamically selects a JSON implementation from a list of
candidates. Since we cannot control which gems are installed on a user's
machine, this patch hardwires `multi_json` to use the included copy of `ok_json`.

`ok_json` is a pure-Ruby JSON encoder/decoder that is bundled with
`multi_json`. `ok_json` may not be as fast as other choices, but speed is not
critical for our application.

Closes Homebrew#8574.

Signed-off-by: Charlie Sharpsteen <source@sharpsteen.net>

Library/Homebrew/vendor/multi_json.rb

@@ -22,18 +22,6 @@ def engine
   # if any engines are already loaded, then checks
   # to see which are installed if none are loaded.
   def default_engine
-    return :yajl if defined?(::Yajl)
-    return :json_gem if defined?(::JSON)
-
-    REQUIREMENT_MAP.each do |(library, engine)|
-      begin
-        require library
-        return engine
-      rescue LoadError
-        next
-      end
-    end
-
     :ok_json
   end
 
@@ -47,7 +35,7 @@ def default_engine
   def engine=(new_engine)
     case new_engine
     when String, Symbol
-      require "multi_json/engines/#{new_engine}"
+      require "vendor/multi_json/engines/#{new_engine}"
       @engine = MultiJson::Engines.const_get("#{new_engine.to_s.split('_').map{|s| s.capitalize}.join('')}")
     when Class
       @engine = new_engine

Library/Homebrew/vendor/multi_json/engines/ok_json.rb

@@ -1,4 +1,4 @@
-require "multi_json/vendor/ok_json" unless defined?(::OkJson)
+require "vendor/multi_json/vendor/ok_json" unless defined?(::OkJson)
 
 module MultiJson
   module Engines