do not fail if handler does not respond to payload

lib/travis/addons/instrument.rb

@@ -23,7 +23,7 @@ def publish(event = {})
           :event => handler.event
         )
 
-        event[:payload]    = handler.payload
+        event[:payload]    = payload
         event[:request_id] = request_id
         event[:repository] = repo
         super(event)
@@ -40,6 +40,10 @@ def serialize(object)
           "#<#{object.class.name} #{pairs.map { |key, value| [key, value].join('=') }.join(' ')}>"
         end
 
+        def payload
+          handler.payload if handler.respond_to?(:payload)
+        end
+
         def repo
           object.repository if object.respond_to?(:repository)
         end