'no matching distribution found for cffi' error on provisioning

Issue Background

mungaip has reported a provisioning failure in B@D - install_dependencies.sh fails with the error 'no matching distribution found for cffi'

Issue description

I don't see this error here and am investigating wither this is vagrant/virtualbox version dependent

Acceptance criteria

• When the B@D environment provisions with vagrant 1.9.1 - the version specified in the wiki prerequisites.
• Any necessary config changes to pip have been tested in the proxy-less configuration.
• An item has been added to know issues reporting a range of possible methods when working with a proxy.