logs cause exception with non-ascii characters

[tgermain]

#How to reproduce
Create an app with non-ascii character in labels for exemple :

{
  "id": "nonascii",
  "mem": 32.0,
  "cpus": 0.5,                   
  "instances": 1,
  "labels": {
    "nonascii":"ççççç"
  },
  "container": { 
    "docker": {
      "portMappings": [
        {
          "servicePort": 0,
          "protocol": "tcp",
          "containerPort": 443,
          "hostPort": 0
        },
        {
          "servicePort": 0,
          "protocol": "tcp",
          "containerPort": 80,
          "hostPort": 0
        }
      ], 
      "image": "nginx",
      "network": "BRIDGE",       
      "priviledged": false
    },
    "type": "DOCKER",
    "volumes": []
  },
  "healthChecks": []
}

If you try to use get_apps() you will raise an exception : UnicodeEncodeError: 'ascii' codec can't encode characters in position 95-96: ordinal not in range(128)

The tricky part is that the actual call to marathon achieve through requests here succeed. But if response.text contains any non-ascii char, this call :

marathon.log.error('Got HTTP {code}: {body}'.format(
code=response.status_code, body=response.text))

will fail.

See logs starting form here : https://github.com/thefactory/marathon-python/blob/master/marathon/client.py#L93