So it looks like Apple have removed the discoveryd and discoveryutil commands from terminal.

To flush your DNS on OS X 10.11 El Capitan, issue this command in terminal:

sudo dscacheutil -flushcache;sudo killall -HUP mDNSResponder;say flushed 

---

Shouts to http://blog.chlaird.com/2015/06/os-x-1011-el-capitan-flush-dns.html for the info.