Updated nokogiri to 1.6.0


Hi, first of all thanks for the great work! We've been using savon for years!

I'm not sure if there are any known issues regarding this update. I've had a quick look on the closed ones and found nothing.

So, I updated nokogiri because to be stuck with 1.5.10 is really annoying every time you run bundle update if you also use cucumber.

Again, I'm not sure if this working properly, but the specs are OK and, I've already ran the savon specs (including integration ones) without problems  here: https://github.com/rafamanzo/savon/tree/version2-nokogiri-1.6
So what do you think?

Thanks again and sorry if I'm completly wrong!