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.

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