Update json to 1.8.3

Make it bundle on OS X with Ruby 2.3 from brew.

Author: boltomli

Gemfile.lock

@@ -22,7 +22,7 @@ GEM
       nokogiri (~> 1.6.1)
       rubyzip (>= 1.1.1)
     hashery (2.1.1)
-    json (1.8.1)
+    json (1.8.3)
     kindlegen (2.9.4)
     mini_portile (0.6.0)
     nokogiri (1.6.3.1)