Don't rescue signals when running experiments

`rescue Object` is equivalent to `rescue Exception`, which will catch signals and other messages which shouldn't be silenced during an experiment.

See github#60

lib/scientist/observation.rb

@@ -26,7 +26,7 @@ def initialize(name, experiment, &block)
 
     begin
       @value = block.call
-    rescue Object => e
+    rescue StandardError => e
       @exception = e
     end