indirect installer: Use curl instead of wget

wget 1.12 on Chromium OS < 5116 (R33) is buggy and not able to fetch the latest installer.

Also, use direct URL as curl does not follow redirections.

Fixes #1033 for good, so we can leave the version check in the main installer.
In that case, should we switch all of crouton from wget to curl?  Looking through rootfs, it seems like Chromium OS uses curl and not wget, which makes wget ripe for removal at any point...