[bug 1116543] Peg pip at 1.5.6

pip 1.5.6 works with the version of peep we have. We peg pip at 1.5.6 so
that vagrant provisioning works correctly.

We can unpeg this when peep works with the latest version of pip which
is like 1.6.something.
Merge pull request #426 from willkg/1116543-fix-peep

[bug 1116543] Peg pip at 1.5.6