Added Mesos/Marathon requests v1/v2 compatibility

# Problem
Marathon check was written for requests v1.x api

# Solution
I added a condition to return if `r.json` is a dict else return the response from the callable method

## Example of error
```python
>>> import requests
>>> r = requests.get('http://localhost:8080/v2/apps')
>>> r.json
<bound method Response.json of <Response [200]>>
>>> r.json()
{u'apps':
```

# Tests
- [x] Tested datadog info
```
  Checks
  ======
  
    mesos
    -----
      - instance #0 [OK]
      - Collected 115 metrics, 0 events & 0 service checks
  
    marathon
    --------
      - instance #0 [OK]
      - Collected 141 metrics, 0 events & 0 service checks
  
    network
    -------
      - instance #0 [OK]
      - Collected 11 metrics, 0 events & 0 service checks
  
  
  Emitters
  ========
  
    - http_emitter [OK]
```

# Notes
- Since I added the condition to marathon I went ahead and added it to mesos for consistency