Bug/cause

[nobu]

Since ruby 2.6, Exception#cause is displayed at error exit.

$ ruby2.6 -ripaddr -e 'p IPAddr.new("quax")'
Traceback (most recent call last):
	3: from -e:1:in `<main>'
	2: from -e:1:in `new'
	1: from /opt/local/lib/ruby2.6/2.6.0/ipaddr.rb:586:in `initialize'
/opt/local/lib/ruby2.6/2.6.0/ipaddr.rb:651:in `in6_addr': invalid address (IPAddr::InvalidAddressError)
	3: from -e:1:in `<main>'
	2: from -e:1:in `new'
	1: from /opt/local/lib/ruby2.6/2.6.0/ipaddr.rb:557:in `initialize'
/opt/local/lib/ruby2.6/2.6.0/ipaddr.rb:598:in `rescue in initialize': invalid address: quax (IPAddr::InvalidAddressError)

This is completely duplicate and unnecessary.

[ioquatix]

Merged @jeremyevans suggested fix.