Massachusetts is inching closer to the official launch of its casino industry, as Penn National Gaming submitted revised plans Thursday for a slot parlor slated to open in June along the Rhode Island border while regulators highlighted steps the state still needs to complete before it can oversee gambling operations. Plans for Plainridge Park still call for a gambling floor with 1,250 slot machines but other parts of the $225 project have changed slightly, according to a report submitted to the Massachusetts Gaming Commission.