network retry incase nginx hasn't bound to the port yet

These numbers arrived at by wildly unscientific methods involving apachebench.

Much thanks to @dzuelke for the template, via the PHP buildpack.

With `set -e` we don't need to use && to control sequencing.

I've written this up to use a heroku-specific config file (`backends.yml`,
which is optional) rather than nginx config so that in principle we can move it
to implementations other than nginx.