Changes things so the latest ruby and varnish are not required.

Went to XML rather than JSON output from varnishstat so that older versions are supported (ie. 3.0.2). Also had to adjust the command object passed to popen() for ruby 1.8.7.
Prevent bundler/setup errors

Fixes "./newrelic_varnish_plugin:35:in `require': no such file to load
-- bundler/setup (LoadError)" errors on startup.