update default_gateway.rb module to ruby 1.9

while testing puppet3 + trusty in labs I noticed that facter was failing on
this because ruby 1.9 removed "to_a" method from strings. This is a minimal
change to match what upstream has at
https://github.com/kwilczynski/facter-facts/blob/master/default_gateway.rb

It is backwards compatibile with 1.8 and 1.9 from my testing:

default_gateway => 10.68.16.1
default_gateway_interface => eth0
rubyversion => 1.8.7

default_gateway => 10.68.16.1
default_gateway_interface => eth0
rubyversion => 1.9.3

Change-Id: I4c623077d95976eb1a5aff5710e23e04899506d2