Change activation of conda environments in powershell

[DonJayamanne]

Once the upstream Conda Issue #626 is resolved, we'll need to update the conda activation accordingly.

[kalefranz]

A contributed powershell wrapper to conda would be hugely appreciated. With conda 4.4+, there are five other shell wrappers for conda. We're still obviously missing powershell.

I started the conda/conda#6471 PR as a sketch of what the powershell wrapper might look like, and Steve Dower added some additional comments in conda/conda#6471 (comment).

Eager to work with anyone wanting to push this through and get it in.