tox fails with pip 10

Trying to run tox in a fresh stem workspace (no .tox dir, which contains the virtualenvs) now fails.

Failure output with pip 10:

no such option: --allow-all-external
ERROR: InvocationError: 'path/to/pip install --allow-all-external -e .'

Here's a deprecation note from a tox / pip 9 virtualenv:

py27 runtests: commands[0] | pip install --allow-all-external -e .
DEPRECATION: --allow-all-external has been deprecated and will be removed in the future. Due to changes in the repository protocol, it no longer has any effect.

With pip 10 (somewhat recently released), this --allow-all-external flag seemingly got removed.

**atagar: I'll have a patch for this shortly! **

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