older-nginx-versions: Add NGX_CONF_MULTI to ps_command flags.

Older nginx versions don't allow 'pagespeed' configuration
directives to appear in blocks other then http{} without it.

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/786
Merge pull request #798 from pagespeed/oschaaf-trunk-tracking-conf-multi

older-nginx-versions: Add NGX_CONF_MULTI to ps_command flags.