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.

lib/nanoc/cachebuster.rb

@@ -35,7 +35,7 @@ def self.should_apply_fingerprint_to_file?(item)
     end
 
     def self.fingerprint_file(filename, length = 8)
-      CACHEBUSTER_PREFIX + Digest::MD5.hexdigest(File.read(filename))[0..length.to_i]
+      CACHEBUSTER_PREFIX + Digest::MD5.hexdigest(File.open(filename, 'rb'){|io| io.read})[0..length.to_i]
     end
   end