pipenv shell command fails on windows

`pipenv-shell` fails on windows due to:

```
'exec' is not recognized as an internal or external command,
operable program or batch file.
```

Relevant line: https://github.com/pwalsh/pipenv.el/blob/b730bb509e8b60af9f5ab1f1e6c3458d1d95d789/pipenv.el#L70

