Unicode bug

I went with `env = {str(key): str(value) for key, value in env.items()}` as it is robust to any future additions to the `env` dictionary.
If you want to remove this and have `MetaData.__unicode__` still work on Python 2, you're going to need to change:

```python
def __unicode__(self):
        '''
        String representation of the MetaData.
        '''
        return str(self.__dict__)
```

to 

```python
from conda.compat import text_type
def __unicode__(self):
        '''
        String representation of the MetaData.
        '''
        return text_type(self.__dict__)
```