无法更改Codeigniter验证码代码字符串序列

I am using codeigniter 2.1.4. I am using its captcha helper. It works perfectly fine. But i do not like the representation of characters image string. The image string is in wavy order. This is what i am getting:

I want the captcha string characters to be displayed in line like below image.

How can i achieve this? Please help. Thanks in advance.