Robinhood, the free stock-trading app with 21 million active users and counting, is about to hit the road for a college coffeehouse tour to drum up new customers.Now where have we heard this one before? Ah, yes, the credit card industry.The campus antics that the card companies got up to two decades ago were so egregious that they helped lead to a 2009 federal law that made it harder for anyone under 21 to get their products in the first place.There are some important differences. Credit card issuers can put marks on your record that can keep you from qualifying …