Thankfully this feature is available in the free version. Nova recently added this feature in the launcher as Beta. Not all devices support this feature but it works on Atrix 2 with ease. I have tried it and it really works. I have also tried this feature on my cousin's MotoE but it din't work.
To turn on the voice command, first install and set the Nova Launcher as default launcher with 'always' option. Reboot after installation will be better. Now you need to turn on the 'Ok Google' hot-word detection in the Nova settings.
Go into Nova setting > tap on the “OK, Google” option > Turn the check box on and you are done.
Now you can control your Atrix2 without touching it like Moto X.
And do note that its a Google feature which won't work without internet connection.
Those who haven't used Nova launcher yet should know that this is one very lightweight, simple and fluid launcher yet provides lot of customization. It also comes as default launcher in Slim ROMs. Nova looks simple at first look but there are limitless possibilities of many other customization in the Nova once you understand what option does what. You can change icons, colours, animations and much more without straining the phone too much.
So let's enjoy the smartest feature of MotoX in the Great His Highness Atrix 2. There is no limit with this epic phone. Mu-ha-haa-haaa