Update cachebuster to open files in binary mode

It seems there's an issue where the fingerprint of a PNG file will always be 658e8dc0bf8b9a09b36994abf9242099, due to the way File.read works (see https://groups.google.com/forum/#!topic/asciidoc/SUsR8GcqZ08). This commit changes fingerprint_file to open the file in binary mode.