Use IPAddr class to validate ip formats

[RST-J]

This fixes #133 and also uses the IPAddr class to validate ipv4.

[pd]

Eek; while wading through the older issues that were still open, I ran across this comment about why IPAddr was avoided: #79 (comment)

@hoxworth what are your thoughts on this? Was it an issue on 1.8.x only? We don't run travis on 1.8 right now; does that mean it's no longer supported?

[iainbeeston]

When I set up Travis I didn't include Ruby 1.8 because (I believe) it's no longer maintained or updated.

[iainbeeston]

Also, if json-schema should be 1.9+ (or 1.8+) we should update the gemspec to say that, explicitly.

[hoxworth]

@pd I'm not as concerned as I used to be over the system callout for IPAddr.new - I'd much rather use the built-in libraries, and I am comfortable moving to the built-in libraries at this point, even for 1.8.7. I'd like to maintain compatibility with 1.8.7 at the very least, as there are still some people using that version, but certain capabilities are starting to make me want to drop 1.8x support...