One way is to create a transparent graphic and set it as the background image of your Backbutton.
So your code should look like this:
let backButton = UIBarButtonItem(title: "< back", style: UIBarButtonItemStyle.Plain, target: self, action: nil)
backButton.setTitleTextAttributes([NSFontAttributeName: UIFont(name: "RobotoCondensed-Light", size: 20)!], forState: UIControlState.Normal)
backButton.setBackButtonBackgroundImage(UIImage(named: "transparent_1px.png"), forState: UIControlState.Normal, barMetrics: UIBarMetrics(rawValue: 0)!)
navigationItem.backBarButtonItem = backButton