Update addto-startup.py: fix empty working directory in OSX

WorkingDirectory in plist generated by this script is empty. and proxy.py can't be started in Yosemite:

com.apple.xpc.launchd[1] (org.goagent.macos[1524]): Service could not initialize: Unable to set current working directory. error=2, path=: 14A389: xpcproxy + 12907 [1227][1016C726-9ACF-3A24-9C51-A279F5C6B167]: 0x2

this fix set the WorkingDirectory to correct folder.