Previously, we installed the latest version of pip and virtualenv.
This commit pins the pip and virtualenv version so we install known
working versions (pip 10 breaks the image build for some reason).
MozReview-Commit-ID: hOAMencdcr
--HG--
extra : rebase_source : 2cb44c2ef55e29c55cf3d1b354e90d6fb5414cce