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.
Yeah, we can do this. I thought this way, we don't have to touch the archives at all. But it's okay, we need to have some documentation for the poor soul who needs to do this the next time. ;-)