DP-132 fallback on localhost.localdomain if host's hostname is an ipaddr.

Otherwise postfix will fail to start, emitting an error message like:

postfix: warning: valid_hostname: numeric hostname: 1.2.3.4
postfix: fatal: file /etc/postfix/main.cf: parameter myhostname: bad parameter value: 1.2.3.4
DP-132 Replace ip check with something built into ruby.
Merge pull request #4 from VertiPub/jb-DP-132

DP-132 fallback on localhost.localdomain if host's hostname is an ipaddr