keyboard - How to run command at startup?

The Return key in my keyboard is broken, so I have chosen to make my PgDn key to act like the Return key. Now I learned how to do this using 'xmodmap'. The command I have to run is:

xmodmap -e 'keycode 117 = Return'

But I have to run this command everytime I reboot. I googled around and found that placing this command in the /etc/rc.local file should run it automatically at start-up. However, it does not work.

Can anyone tell me how can I execute this command when I boot up? I am using Ubuntu 14.04.