bug#18524: Guix unable to download from github due to TLS fatal alert

[Ludovic Courtès]

I had managed to bork ‘set-session-server-name!’ in GnuTLS (which
addresses this bug), so this is fixed in commit 5186158 and upstream:
<https://gitorious.org/gnutls/gnutls/commit/e4e513f43a8bdd9fe50bdd95a7fd213bca2f81b0>.

Sorry about that!

Ludo’.