'-format aces' flag does not produce valid image files

[scottdyer]

Something appears to have broken in the '-format aces' output option.

ctlrender does not return an error, and the file appears to have been written successfully. However, the resulting file is unrecognized by both the operating system and by OpenEXR utilities such as exrheader.

I am on a Mac running OS 10.9.1, running the following:
-- found IlmBase, version 2.0.1
-- found TIFF, version 4.0.3
-- Found OpenEXR, version 2.0.1
-- Found AcesContainer, version 1.0.0