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.
it isn't clear where the symbol files are being written from this.  The windows location should be `cwd()/deps/cef/release/` and `cwd()/deps/cef/release/` respectively.  I don't think Mac needs a special location, you can store them wherever since your only option is to symbolize crash dumps with the symbols.