Avoid Ruby 1.8 compatibility when using Ruby 2.1.8

[wpiekutowski]

There's a small problem in https://github.com/bluemonk/ipaddress/blob/master/lib/ipaddress.rb#L217.
This regular expression is matching not only Ruby 1.8, but also 2.1.8. Because of that, when you upgrade to any Ruby with a version similar to x.1.8, you'll get a ton of warnings similar to:

.rbenv/versions/2.1.8/lib/ruby/gems/2.1.0/gems/sprockets-2.11.3/lib/sprockets/mime.rb:25: warning: Hash#index is deprecated; use Hash#key

That code is correctly using Hash#key, but due to the alias made in

https://github.com/bluemonk/ipaddress/blob/master/lib/ipaddress.rb#L219

Hash#key becomes an alias to Hash#index and actually Hash#index is called, which results in a lot of depreciation warnings.

[wpiekutowski]

Would it be possible to release 0.8.3 with only this small fix? If so, I'm happy to create a proper branch based on 0.8.2.

[bluemonk]

What a silly bug! Good catch, thanks for the PR.
I will release 0.8.3 now.

[wpiekutowski]

Awesome, thank you for a quick reaction.