Marathon >= 0.7 failing

[amvapor]

The Marathon plugin checks the TaskRateLimit attribute from the Marathon 0.6 API version, which has been removed https://github.com/mesosphere/marathon/blob/master/docs/docs/upgrade/06xto070.md#task-rate-limit

you can see the dependency here:

dd-agent/checks.d/marathon.py

Line 30 in 73dd750

for attr in ['taskRateLimit','instances','cpus','mem','tasksStaged','tasksRunning']:

using marathon v0.7.6, the dd-agent info looks like this:

    marathon
    --------
      - instance #0 [ERROR]: KeyError('taskRateLimit',)
      - Collected 1 metric, 0 events & 1 service check