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.
Does this work if we use curl -L?  Github apparently has raw., raw2., etc, and I'd rather not hardcode it.