Detroit billionaire Dan Gilbert celebrated his $1.8 billion Rocket Companies IPO Thursday by talking up the possibility of acquiring more financial-technology companies down the road.“We want to use our stock as currency and potentially acquire more fin-tech organizations and put them in the vault,” Gilbert said on CNBC. “We’re excited about it.”Rocket stock began trading on the New York Stock Exchange Thursday around 11 a.m. opening at the IPO price of $18 a share. The stock was gaining ground and trading around $18.65 a share shortly after 11:30 a.m. In early trading, the stock swung from a …