Adding the gemspec file from the 1.6.3 version of the nokogiri gem so
that we can install the gem from our Github branch without LOAD_PATH
issues (hoepefully).

Why doesn't Nokogiri have a gemspec in it's repo ?  Read about that
here:  https://github.com/sparklemotion/nokogiri/blob/master/Y_U_NO_GEMSPEC.md