Create `/etc/ssl/cert.pem` symlink on FreeBSD

This is the default location most Unix tools look for the default root
CA cert. The `ca_root_nss` port (which creates `ca-root-nss.crt`)
also optionally creates this symlink but we cannot gurantee this was
done at port install time.
Merge pull request #77 from opscode-cookbooks/schisamo/freebsd-ca-cert-symlink

Create `/etc/ssl/cert.pem` symlink on FreeBSD