Download CEF builds from S3

Story https://trello.com/c/1JK3Ordd

Changed the download URL to get the CEF builds from S3 instead from dev.brackets.io.
Added task to download the symbol files for the CEF version that we use to build Brackets.
oh i would repackage as a zip file.  We do this already with the other 7z files -- we just haven't done symbols yet. When I give you the CEF binaries, I get them as 7-zip files then recompress them with WinZip into zip files.  Let's just do that for symbols so we don't have to worry about compatibility with whatever library or command line tool you're using.