module RecaptchaMailhide
  class Configuration
    attr_writer :private_key, :public_key

    def private_key
      raise "RecaptchaMailhide's private_key is not set. If you're using Rails add an initializer to config/initializers." unless @private_key
      @private_key
    end

    def public_key
      raise "RecaptchaMailhide's public_key is not set. If you're using Rails add an initializer to config/initializers." unless @public_key
      @public_key
    end
  end
end

module RecaptchaMailhide
  class Configuration
    attr_accessor :private_key, :public_key
  end
end

--- 
+++ 
@@ -1,5 +1,15 @@
 module RecaptchaMailhide
   class Configuration
-    attr_accessor :private_key, :public_key
+    attr_writer :private_key, :public_key
+
+    def private_key
+      raise "RecaptchaMailhide's private_key is not set. If you're using Rails add an initializer to config/initializers." unless @private_key
+      @private_key
+    end
+
+    def public_key
+      raise "RecaptchaMailhide's public_key is not set. If you're using Rails add an initializer to config/initializers." unless @public_key
+      @public_key
+    end
   end
 end
Make private_key and public_key raise exceptions if missing from config.
