No need to download a file when comparing the MD5 hash

Originally the file was needed to compare the MD5 by calculating it. Later, it was found the hash exists on the file's header. Found by @coen-hyde here https://github.com/geedew/grunt-s3/commit/fde0f93918437129b303420a3f66d75a4cf3fc7f#commitcomment-3868274