Update for pyenv environment creation on OSX

I have tried the pyenv environment setup option in #122 but it did not work for me. I was not able to generate the shared library and PyCall was giving the error

```
ImportError: No module named site
```

Instead, the following command worked

```
env PYTHON_CONFIGURE_OPTS="--enable-shared" pyenv install 3.6.5
```

The difference is to use `--enable-shared` instead of `--enable-framework`. This is also described [here](https://github.com/pyenv/pyenv/wiki#how-to-build-cpython-with---enable-shared).

Review the pyenv environment-setup instructions referenced by #122 and compare them with the reported working command on macOS. Done when the instructions use the appropriate option and explain the resulting PyCall setup without the reported ImportError.

Scritto dal modello di indicizzazione a partire dal testo della issue.