Replace use of #fail with #raise

Author: rastating

lib/wpxf/wordpress/reflected_xss.rb

@@ -17,7 +17,7 @@ def initialize
   # @return [Boolean] true if successful.
   def run
     unless respond_to? 'url_with_xss'
-      fail 'Required method "url_with_xss" has not been implemented'
+      raise 'Required method "url_with_xss" has not been implemented'
     end
 
     return false unless super