discoveryutil: command not found

Solution 1:

Since discoveryutil was replaced by its predecessor mDNSResponder in 10.10.4 use sudo killall -HUP mDNSResponder again (like in Lion/Mountain Lion/Mavericks) to flush the DNS cache.

Solution 2:

A colleague has suggested sudo dscacheutil -flushcache. The Apple documentation states

dscacheutil -- gather information, statistics and initiate queries to the Directory Service cache.

-flushcache Flushes the entire cache. This should only be used in extreme cases. Validation information is used within the cache along with other techniques to ensure the OS has valid information available to it.