Upgrade gnutls recipe to install 3.1.8 to include an important bugfix

The bug fix can be found here:
http://article.gmane.org/gmane.comp.encryption.gpg.gnutls.devel/6623

This bug has many side effects on other applications that require gnutls. In my case, I had an issue with bitlbee and connected to jabber (gtalk) as it requires a tls connection. Upgrading to 3.1.8 fixes this issue and many other ones that are related to the bug in 3.1.7.