Developer docs: version-lock Ansible under OS X
Developer docs: version-lock Ansible under OS X

The OS X setup docs now specify an explicit Ansible version, same as
the Ubuntu/Debian docs. We shouldn't be recommeding use of `sudo` for
the pip installation, so added a TODO to address that.

conorsch/securedrop,heartsucker/securedrop,micahflee/securedrop,garrettr/securedrop,ageis/securedrop,ehartsuyker/securedrop,conorsch/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,garrettr/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,ageis/securedrop,micahflee/securedrop,heartsucker/securedrop,micahflee/securedrop,conorsch/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,ageis/securedrop,micahflee/securedrop,ehartsuyker/securedrop,ageis/securedrop,ehartsuyker/securedrop,garrettr/securedrop,conorsch/securedrop,garrettr/securedrop