New download paths

Since mesos 0.19.0 the download url for the deb and rpm packages has changed. This PR changes it accordingly and bumps the default mesos version to the current 0.20.1.
I'm open to that idea since we don't have any dependency on < 0.19. It would be nice to have that support though. @MrMarvin are you inclined to add the backwards-compatibility support for the old URIs?