Package using deprecated eth-util methods.

* Populus Version: 2.2.0
* OS: linux

### What was wrong?

Populus attempts to import deprecated methods from `eth-utils`:

```shell
File "/usr/local/lib/python3.6/site-packages/populus/utils/mappings.py", line 8, in
from eth_utils import (
ImportError: cannot import name 'force_text'
```

This error is encountered when attempting to run `pytest` from the https://github.com/gitcoinco/web/ codebase with `eth-utils==1.0.1`

Relates to: https://github.com/ethereum/eth-utils/pull/69/

I'm uncertain whether or not you'd prefer a compat module to handle `ImportError` exceptions for backwards compatibility or outright changes to the existing imports.

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