Replace deprecated entry in launchd config with the modern equivalent.

From `man launchd`:

>This key was used in Mac OS X 10.4 to control whether a job was kept alive or not. The default was true.  This key has been deprecated and replaced in Mac OS X 10.5 and later with the more powerful KeepAlive option.

We require OS X 10.6 minimum so changing this should not be an issue.