Prevent error when seeing an exception thrown by Rails.

Hi,

I tried to use Logstasher 0.2.8 with Rails4 (served by Puma & Ruby 2.0.0-p247 ).
When rails throws an exception, Logstasher doesn't catch and log it, instead the following error is raised:

````
E, [2013-10-14T04:14:43.672210 #8396] ERROR -- : Could not log "process_action.action_controller" event. NoMethodError: undefined method `<<' for nil:NilClass ["/var/www/app/capistrano/current/vendor/bundle/ruby/2.0.0/gems/logstasher-0.2.8/lib/logstasher/log_subscriber.rb:17:in `process_action'", 
````

The change I made fixes the issue and allows the Rails exception to be rendered in Logstasher Json output file.