Update nokogiki dependency >= 1.5

Following http://guides.rubygems.org/patterns/

A previous update forcing only Nokogiri 1.6 breaks several
applications.  Since Nokogiri 1.5 still works just fine I
modified the dependency to be >= 1.6 which accounts for
Nokogiri 1.6 as well.
Merge pull request #19 from sag47/feature_better_nokogiri_support

Update nokogiri dependency >= 1.5