cloudpickle breaks isinstance

Follow up to https://github.com/dask/distributed/issues/2228

```python
In [1]: import cloudpickle

In [2]: class MyObject(object):
   ...:     pass
   ...: 
   ...: 

In [3]: x = MyObject()

In [4]: x2 = cloudpickle.loads(cloudpickle.dumps(x))

In [5]: isinstance(x2, MyObject)
Out[5]: False
```

Using `cloudpickle==0.5.5`, Python 2.7.12, Ubuntu 16.04.