Proper use of encoding for hashing international characters.

The [documentation for hash.update()](http://nodejs.org/api/crypto.html#crypto_hash_update_data_input_encoding) states that if no encoding is given, the argument is expected to be a Buffer.

Like many other "hash tasks" for Grunt, grunt-rev-md5 [provides the contents of the file as a string](https://github.com/tactivos/grunt-rev-md5/blob/master/tasks/grunt-rev-md5.js#L104) to `hash.update` without any encoding as a second argument, which is incorrect.

The result of this is an incorrect hash for files with international characters in them when no encoding is given to `hash.update`.

Setting the proper encoding when calling `hash.update` fixes this.

PS. This PR is untested as the plugin is not updated for Grunt 0.4. But you get the gist :)