Avoid duplicate files in remote files list cache (#439)

Files on the "always_update" list would show more than once if duplicate
entries aren't removed.

After the first upload, files that are in the `always_upload` list would
start showing up more than once on the list of files. This doesn't cause
a real problem but the list of files is not really accurate. Adding this
`uniq` solves the problem.

lib/asset_sync/storage.rb

@@ -108,7 +108,7 @@ def update_remote_file_list_cache(local_files_to_upload)
       return if ignore_existing_remote_files?
 
       File.open(self.remote_file_list_cache_file_path, 'w') do |file|
-        uploaded = local_files_to_upload + remote_files
+        uploaded = (local_files_to_upload + remote_files).uniq
         file.write(uploaded.to_json)
       end
     end