Change build badge to new master pipeline.
Change build badge to new master pipeline.

Summary: The old badge was referring to a deprecated Jenkins job.

Test Plan: n/a

Reviewers: meichstedt, aquamatthias, zen-dog

Reviewed By: aquamatthias, zen-dog

Subscribers: marathon-team, jenkins

Differential Revision: https://phabricator.mesosphere.com/D436

janisz/marathon,gsantovena/marathon,gsantovena/marathon,meln1k/marathon,Caerostris/marathon,Caerostris/marathon,meln1k/marathon,mesosphere/marathon,janisz/marathon,Caerostris/marathon,janisz/marathon,meln1k/marathon,Caerostris/marathon,guenter/marathon,natemurthy/marathon,natemurthy/marathon,mesosphere/marathon,mesosphere/marathon,natemurthy/marathon,guenter/marathon,mesosphere/marathon,guenter/marathon,janisz/marathon,gsantovena/marathon,meln1k/marathon,gsantovena/marathon,guenter/marathon,meln1k/marathon,guenter/marathon,gsantovena/marathon,mesosphere/marathon,janisz/marathon,Caerostris/marathon