Bug Fix: Cannot define constraints with a tuple of strings

This PR is a very simple fix.  Currently, if a user attempts to define the following situation:

```
dct = dict(constraints=[['hostname', 'UNIQUE']],    << plus other key:value pairs >> )
client.create_app(**dct)
```

The marathon client raises the following exception:

```
  File "/opt/anaconda/2.0.0/lib/python2.7/site-packages/marathon/client.py", line 88, in create_app
    data = json.dumps(app.json_encode())
  File "/opt/anaconda/2.0.0/lib/python2.7/site-packages/marathon/models/app.py", line 84, in json_encode
    'constraints': [c.json_encode() for c in self.constraints],
AttributeError: 'unicode' object has no attribute 'json_encode'
```