Use Sinatra::IndifferentHash instead of indifferent_hash due to Sinatra update (#31)

Bug introduced by #24, when we updated Sinatra from 1.4.7 -> 2.0.1, but missed updating
the json content-type branch.

Fixes #29, alternative to #30 (don't need to maintain `indifferent_hash` backwards compatibility because we specify the Sinatra version in the Gemfile)

Author: danj-stripe

web.rb

@@ -42,7 +42,7 @@ def log_info(message)
   # Get the credit card details submitted
   payload = params
   if request.content_type.include? 'application/json' and params.empty?
-    payload = indifferent_params(JSON.parse(request.body.read))
+    payload = Sinatra::IndifferentHash[JSON.parse(request.body.read)]
   end
 
   source = payload[:source]