improve: since no instance variable will be used in block, instance_eval is better than instance_exec accoding to the semantic.

Author: 叶思杰

lib/exception-track.rb

@@ -22,7 +22,7 @@ def config
     end
 
     def configure(&block)
-      config.instance_exec(&block)
+      config.instance_eval(&block)
     end
   end
 end