Python 3.6.0a4

[JayH5]

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[JayH5]

For whatever reason pip isn't available as pip in 3.6.0a4, only as pip3, which is causing the image to fail to build 😢

[tianon]

Aha! We finally found the edge case that led to adding --ignore-installed! (#121, #118, #100)

Looks like Python 3.6.0a4 already comes with the latest version of pip, so it no-ops when we ask it to install, which means it doesn't create the pip symlink properly. 🤘

I think --force-reinstall is the option we really want (instead of --ignore-installed; http://stackoverflow.com/a/19549035) to ensure it cleans up references to the older versions properly (which is what that pip list line is verifying). If you'd like to try taking that change on, you're welcome to, otherwise I'm happy to take over from here (and think we'd be fine to merge this PR as-is, since the version bump is correct even if the build is currently failing). 👍

[JayH5]

@tianon you can go ahead and take over 😄. --force-reinstall sounds about right (although maybe with a comment this time so that we remember why 😛)