graylog2-server plist: Set KeepAlive to true

graylog2-server can't start if elasticsearch isn't running. Although I have a launchd plist file for elasticsearch it needs a few seconds to start and therefore get the following error:

syslog:

    3/30/13 9:42:16.815 PM com.apple.launchd.peruser.501[183]: (homebrew.mxcl.graylog2-server[286]) Exited with code: 1

graylog2 log:

    2013-03-30 21:42:16,757 FATAL: org.graylog2.Main - IOException while trying to check Index. Make sure that your ElasticSearch server is running.

Setting ```KeepAlive``` to ```true``` makes launchd retry every 10s and gives elasticsearch time to get running.