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.
is this where the zip files are unzipped to?  This is fine for Mac but, for Windows, it needs to be `deps/cef/Release` for release symbols and `deps/cef/Debug` for debug symbols