limit json version on lower rubies

JonRowe · JonRowe · commit 24f6e5a6e2d8 · 2016-07-03T18:49:05.000+10:00
diff --git a/Gemfile b/Gemfile
@@ -27,7 +27,7 @@ group :documentation do
 end
 
 platforms :ruby_18, :jruby do
-  gem 'json'
+  gem 'json', '< 2.0.0'
 end
 
 platforms :jruby do
