feeds: switch LuCI to Github repo

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 42865

feeds.conf.default

 src-git packages https://github.com/openwrt/packages.git
-src-git luci http://git.openwrt.org/project/luci.git
+src-git luci https://github.com/openwrt/luci.git
 src-git routing https://github.com/openwrt-routing/packages.git
 src-git telephony http://git.openwrt.org/feed/telephony.git
 src-git management https://github.com/openwrt-management/packages.git