Updated nokogiri dependency

Hi,

To get this gem working on my Ubuntu 12.10 machine I had to update the nokogiri version dependency.

The error I was getting on install was:

make
compiling xslt_stylesheet.c
compiling xml_relax_ng.c
compiling xml_xpath_context.c
xml_xpath_context.c: In function ‘xpath_generic_exception_handler’:
xml_xpath_context.c:184:3: error: format not a string literal and no format arguments [-Werror=format-security]
cc1: some warnings being treated as errors
make: *** [xml_xpath_context.o] Error 1