Fix #37 Needed packages for nokogiri added to readme, and while the missing gem detected is nokogiri

lib/environment.rb

@@ -42,6 +42,11 @@ rescue LoadError => e
 
   missing_gem = e.to_s[%r{ -- ([^\s]+)}, 1]
   if missing_gem
+    if missing_gem =~ /nokogiri/i
+      puts
+      puts "Nokogiri needs some packets, please run 'sudo apt-get install libxml2 libxml2-dev libxslt1-dev' to install them. Then run the command below"
+      puts
+    end
     puts "[TIP] Try to run 'gem install #{missing_gem}' or 'gem install --user-install #{missing_gem}'. If you still get an error, Please see README file or https://github.com/wpscanteam/wpscan"
   end
   exit(1)