Error caused by invalid syntax in ACL $override feature

I was running into this via:

S3::putObject(S3::inputFile($file), $bucket, $name, 'ACL_PUBLIC_READ');

that made it along to getACL where $acl = self::$override; would throw an error like:
self::public-read not defined

This fixed it.

Signed-off-by: Ian Turgeon <iturgeon@gmail.com>