Use virtualenv python from install-time for less virtualenv requirements at commit time.

This enables the following workflow:

```
$ sh -c '. venv/bin/activate && pre-commit install'
$ git commit -m ...
```

which previously would have needed to have been:

```
$ sh -c '. venv/bin/activate && pre-commit install'
$ . venv/bin/activate
$ git commit -m ...
```

Closes #164