Notify #marathon-dev when pipeline is green again. (#5274)
Notify #marathon-dev when pipeline is green again. (#5274)

Summary:
If the current build succeeded but the previous failed we notify the Marathon dev
channel on slack.

Test Plan:
pipelines

Reviewers:
aquamatthias, zen-dog

Subscribers: jenkins, marathon-team
natemurthy/marathon,guenter/marathon,janisz/marathon,mesosphere/marathon,mesosphere/marathon,meln1k/marathon,natemurthy/marathon,meln1k/marathon,gsantovena/marathon,guenter/marathon,guenter/marathon,Caerostris/marathon,gsantovena/marathon,gsantovena/marathon,mesosphere/marathon,meln1k/marathon,mesosphere/marathon,janisz/marathon,mesosphere/marathon,janisz/marathon,janisz/marathon,guenter/marathon,Caerostris/marathon,Caerostris/marathon,janisz/marathon,gsantovena/marathon,meln1k/marathon,guenter/marathon,Caerostris/marathon,natemurthy/marathon,Caerostris/marathon,gsantovena/marathon,meln1k/marathon