Update curl to version 7.30.0

This version fixes also a security issue
see http://curl.haxx.se/docs/adv_20130412.html.

The incorporated patch fixes https://sourceforge.net/p/curl/bugs/1219/
and is from upstream.

Signed-off-by: Thomas Braun <thomas.braun@virtuell-zuhause.de>
installer: Minor code simplification

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>