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.
Had a quick look. Unfortunately debootstrap uses wget. So I suggest we do nothing in other parts until wget disappears (if it ever does). Then we can easily modify crouton, and patch debootstrap as needed. But this is not something we want to do unless we are constrained.