I have not tried this yet myself. But looking around for a solution I did find this.
Link http://krazyrobot.com/2014/02/calabash-android-enter-text-from-keyboard-using-adb/ by Balazs Balazs aka krazyrobot
His solution is to use send keycodes, for the home button that is KEYCODE_HOME. Wrapping that into calabash would give you:
system("adb shell input keyevent KEYCODE_HOME")
Again I have not tried this, but it sounds like a solution.