docs(installing_deis): remove xip.io instructions for ELB

xip.io simply doesn't work for ELBs - it returns an IP of
54.64.0.0 which obviously isn't a valid IP.

I suspect it works for CNAMEs in general, but not for CNAMEs
which resolve to multiple IPs, as ELBs do.
Merge pull request #2003 from carmstrong/remove_xip_io

docs(installing_deis): remove xip.io instructions for ELB