The ported Android L Material keyboard works perfectly well on any AOSP based ROMs (Android 4.x.x); however, it requires a rooted phone with custom recovery installed. If everything ok, all you need to do is delete the existing Google keyboard from /system/app and then flash the ported keyboard app from recovery. Installation Instruction:

  1. Uninstall existing Google keyboard or is delete it from /system/app
  2. Download Android L Keyboard from this page (it is a flashable zip file)
  3. Now, boot your phone into custom recovery mode (CWM/TWRP) and flash the downloaded keyboard zip file.
  4. After flashing the file, reboot your phone. Once installed, the new Google keyboard will show up under “Language & input” settings of your phone. To get the Material layout, you need to enable it from the keyboard settings. – Go to Android L Google Keyboard settings -> Advanced settings -> select Color scheme “Material”. Keep in mind that, the layout of this keyboard may not fit perfectly with all phones. (source)