Fix openssl warning for decipher input encoding

Small fix for the following warning:

> node-crypto : Decipher .update encoding can be binary, hex or base64

Changed 2nd arg of [decipher.update](http://nodejs.org/api/crypto.html#crypto_cipher_update_data_input_encoding_output_encoding) to be 'binary' matching the encoding of the input Buffer object. Output remains 'utf8'.