Require a specific and recent version pip
Require a specific and recent version pip

The pip_install command was failing in strange ways
due to variation in the behavior of pip itself
between versions.

For example, older versions don't support the
--no-compile flag.

Also, with the recent versions --download-cache
is no longer supported or needed (its automatic).

R=bradnelson@google.com

Review URL: https://codereview.chromium.org/819843003

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