gemspec: Dont force nokogiri lesser than 1.6.0

Nokogiri drop support for Ruby 1.8 with nokogiri 1.6.0.
It fix a lot of bugs with libxml2 and others.

* https://github.com/sparklemotion/nokogiri/commit/7c5f3f6c444b73192abbcc19ef9e3c987cdf7ab0

And force a user to downgrade his nokogiri version is not very friendly to
a project that want to be up-to-date.

ref #765