simplify dns lookup significantly
simplify dns lookup significantly

dns.lookup does return the address if the argument is an address itself
which makes the checks for an IP address unnecessary. Also we don't need
to explicitly check for an error because we only care if we get an
address.

Tested in 0.10, 0.12 and io.js

sindresorhus/is-reachable,sindresorhus/is-reachable,mmalecki/is-reachable