json gem from github

changed to branch v1.8 repo to avoid that annoying bug 
fbuffer.h > VALUE result = rb_str_new(FBUFFER_PTR(fb), FBUFFER_LEN(fb));
ruby/json#229 (comment)

Gemfile

@@ -1,4 +1,4 @@
 source 'https://rubygems.org'
 gem "sinatra"
 gem "rethinkdb"
-gem "json"
+gem "json", :git => "https://github.com/flori/json.git", :branch => "v1.8"