916666: Displayed 'Next System Check-In' is inaccuarate

GUI now does not show next checkin time when rhsmcertd is not running
update file is now updated each time the frequency is reached so that the checkin
time in the about pane is correct.
Merge pull request #813 from candlepin/wottop/916666

916666: Displayed 'Next System Check-In' is inaccuarate