curl: Update to version 7.29.0.

This version contains a security fix for CVE-2013-0249, for details please have
a look at:

http://curl.haxx.se/docs/adv_20130206.html

Thanks to @roconnor for pointing this out.

I'm pushing this to stdenv-updates because I'm not quite sure if it will break
other things because 7.29.0 is now non-blocking by default. Plus the security
vulnerability only affects IMAP, POP3 and SMTP protocol support.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

Cherry-picked into master, this won't change stdenv.

Signed-off-by: Shea Levy <shea@shealevy.com>