Added --inet4-only to wget calls in coreos-install script.

My server is configured for ipv6 and when I wget the image url I get the following:

~ # wget http://stable.release.core-os.net/amd64-usr/current/coreos_production_image.bin.bz2
--2014-09-23 10:15:12--  http://stable.release.core-os.net/amd64-usr/current/coreos_production_image.bin.bz2
Resolving stable.release.core-os.net (stable.release.core-os.net)... 2a00:1450:4010:c02::80, 74.125.205.128
Connecting to stable.release.core-os.net (stable.release.core-os.net)|2a00:1450:4010:c02::80|:80... connected.
HTTP request sent, awaiting response... 403 Forbidden
2014-09-23 10:15:13 ERROR 403: Forbidden.

Adding --inet4-only to all wget calls fixes this issue.