setup.sh: Try harder to upgrade setuptools

Summary:
Ubuntu precise and Debian wheezy come with an old version of
python-setuptools, which is actually an obsolete fork of setuptools
called 'distribute'.  distribute prevents itself from upgrading to the
original setuptools, i.e. "easy_install 'setuptools>=5.3'" is a no-op.

distribute has since been merged back into setuptools, and new versions
of pip are capable of properly upgrading setuptools, so we must upgrade
pip before upgrading setuptools.

It looks like I broke this in 8f0fa83a3bf3a2fdd9120a42e72e9deffac9b791.

Test Plan:
- Fully provisioning from scratch is still broken, but after some manual
  tweaking of unrelated stuff, "easy_install --version" now returns
  "setuptools 5.7".

Reviewers: charles, spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D423