Update scrypt name to scryptsalsa208sha256.

This function used to be named scryptxsalsa208sha256 but has been
renamed to scryptsalsa208sha2560.

See jedisct1/libsodium@bd05b7d29252365cde4d380bd5ad2671367ff8eb
Merge pull request #108 from thomas-holmes/update-scrypt-name

Update scrypt name to scryptsalsa208sha256.