replace os.popen with subprocess.call in captcha_audio

Hello,

we tried to use apparmor to secure our Django web and we found out that shell (/bin/sh) is executed by your app when flite is run. This is a side effect of using the now deprecated os.popen to run the command and is quite inconvenient when you try to secure the application.
In the attached patch, I replaced it with a subprocess.call function which has the benefit of not starting shell and also taking care of proper argument escaping.

Best regards

Beda