build.sh: Bump DATE to 20140904

build.sh: Bump DATE to 20140904

This pulls in a fix for Layman on Python 3 [1,2].

[1]: https://bugs.gentoo.org/show_bug.cgi?id=521724
     Summary: app-portage/layman-2.1.0-r2 - ImportError: cannot import
       name 'MakeConf'
     Devan Franchini (twitch153) cut layman-2.1.0-r3 with a fix [2].
[2]: http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/app-portage/layman/layman-2.1.0-r3.ebuild?revision=1.1&view=markup
     Summary: Adds patch to fix improper import for ConfigHandler, bug #521724
     Date: 2014-09-02 22:06:59Z