Fix on ruby 2.4

Author: skade

Gemfile.lock

@@ -40,7 +40,7 @@ GEM
       uber (~> 0.0.4)
     http_parser.rb (0.5.3)
     i18n (0.6.11)
-    json (1.8.3)
+    json (1.8.6)
     kramdown (1.4.1)
     listen (2.7.9)
       celluloid (>= 0.15.2)
@@ -135,4 +135,4 @@ DEPENDENCIES
   wdm (~> 0.1.0)
 
 BUNDLED WITH
-   1.11.2
+   1.15.4