Only set PYTHONHOME when using conda

[yuvipanda]

Fixes #410

[yuvipanda]

This is my first Julia piece of code (although @tkelman helped a lot!).

[yuvipanda]

Not sure if this is the best way to do this

[yuvipanda]

I've also not actually tested this on Conda!

[tkelman]

Looking at what Py_SetPythonHome does after we first got this going, maybe it would be easier to do get(ENV, "PYTHONHOME", "") and possibly not need the rest of the patch after this point. Dunno whether a user having the env var PYTHONHOME set to an empty string is sane and should be honored.

[stevengj]

This patch doesn't seem right to me. It seems like we should always call Py_SetPythonHome in PyCall's initialization, since when you do using PyCall you want it to use the python that it was built with and you don't want it to be confused by whatever PYTHONHOME environment variable might happen to be present when you run julia.

It sounds like we might be just setting PYTHONHOME incorrectly in some cases, and the solution is to figure out how to set it correctly?

[iblislin]

It sounds like we might be just setting PYTHONHOME incorrectly in some cases, and the solution is to figure out how to set it correctly?

In case of venv (with py 3.3+), it will work well without setting PYTHONHOME manually, just let it use the default value.

[yuvipanda]

I think #578 supersedes this.