def add_next_block(prev_block, data)
      if valid_block?(prev_block)
        blockchain << next_block(data)
      else
        raise InvalidBlockError
      end
    end
def add_next_block(prev_block, data)
      if valid_block?(prev_block)
        blockchain << next_block(data)
      else
        raise InvalidBlockError
      end
    end
Initialize a new blockchain by creating a new array with the Genesis block

 #add_next_block => Adds a new block to the blockchain with the given data
 it will raise an error if the previous_block hash does not match the last_block in our blockchain