Requre Nokogiri 1.5

If a version isn't specified in the gemspec, 'vagrant plugin install' for vagrant-libvirt will try to install nokogiri 1.6.0. This version of Nokogiri fails to build for me on RHEL6 with the error 

    ./.libs/libxml2.so: undefined reference to `gzopen64'

I think this is because Nokogiri 1.6.0 is bundling a version of libxml2 that requires a newer version of zlib then RHEL6 includes.

This pull request works around the issue by requiring an older Nokogiri which does not bundle libxml2.