Fix AppVeyor - add Scripts to path for current Python

[techtonik]

Tests are failing, because only Python 2.7 is present in PATH

[techtonik]

At least Python 2 is now fully covered, but it looks like pbs is not fully compatible with Python 3.

[techtonik]

I restored pbs project history https://github.com/techtonik/pbs but that's it for now. Need patches.

[spderosso]

Thank you for doing this.

I haven't looked into the pbs code so I am not sure how hard it'd be to make it compatible with Python 3 but it might not be worth the effort. We could say that on Windows we support Python 2 only (and update the appveyor file accordingly).

[techtonik]

Another possibility is to switch to plumbum, which seems to be crossplatform, but syntax needs to be investigated. tomerfiliba/plumbum#332