Added 'l' alias back

The alias for 'ls -la' was removed accidently. I have reverted back the
change so that it is available again. For more information please refer
to
https://github.com/robbyrussell/oh-my-zsh/commit/18cb11903f8208403a0ba5a8f2ece1a0bd4ef9a3#commitcomment-3135898