imagemagick: security bump to version 6.9.0-3

Fixes many of the reported security audit vulnerabilities:
http://www.openwall.com/lists/oss-security/2014/12/24/1



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>