Fix dbf dependency break for Python 2

So the `dbf` package maintainers apparently don't care about versioning and introduced changes in [dbf 0.95.010](https://pypi.python.org/pypi/dbf/0.95.010) that break Python 2 support. Because the `requirements.txt` defines the dependency as `dbf>=0.95.004`, `csvkit` tries to pick up the newer version.

`dbf` attempts to load the [`enum`](https://docs.python.org/3/library/enum.html) package:

```
File "dbf.py", line 67, in <module>

    from enum import Enum, IntEnum

ImportError: No module named enum
```

But that is currently only supported in Python 3.4. So the install just fails. A quick search didn't turn up anything that backports `enum` to Python 2. No bueno.