Revert "README: Update required OpenCV version to 2.4"

This reverts commit 5455ac04d7824bb5a297ddd6aed0f3f8d55b6ae2.

It turns out that it wasn't exactly a bug in OpenCV < 2.4, but incorrect
use of the API on our part. See 4b301f92 for details.
Revert "rpm spec file: Require opencv >= 2.4"

This reverts commit c25d743cbb258ba26695f493a2c6f35181d45d9d.

It turns out that it wasn't exactly a bug in OpenCV < 2.4, but incorrect
use of the API on our part. See 4b301f92 for details.