Check nokogiri gem version before requiring nokogiri

Gives better error messages is using an old nokogiri version

lib/webrat.rb

@@ -10,6 +10,8 @@ module Webrat
   VERSION = '0.3.2.1'
 
   def self.require_xml
+    gem "nokogiri", ">= 1.0.6"
+    
     if on_java?
       # We need Nokogiri's CSS to XPath support, even if using REXML and Hpricot for parsing and searching
       require "nokogiri/css"