Add new versions of wordpress

Hi There!

Just creating this pull request to add the newly released versions of wordpress

<pre>
root@opportinity:~/WhatWeb# for version in `printf "%s\n" 3.8.1 3.8.2 3.8.3 3.9 3.9.1`;do ./whatweb -a 4 -p plugins/wordpress.rb http://localhost/wordpress-$version/; done
http://localhost/wordpress-3.8.1/ [500] WordPress[3.8.1]
http://localhost/wordpress-3.8.2/ [500] WordPress[3.8.2]
http://localhost/wordpress-3.8.3/ [500] WordPress[3.8.3]
http://localhost/wordpress-3.9/ [500] WordPress[3.9.1]
http://localhost/wordpress-3.9.1/ [500] WordPress[3.9]
</pre>