Julia Roberts is flexing her muscles in the rental market. Her "Flatliners" booted "Die Hard 2" out of the No. 1 position on the Billboard magazine rental chart. "Die Hard 2" figured to reign until early next month, when "Ghost" hits the rental chart. But Roberts, the big attraction in "Flatliners," has more rental-market clout than expected. Poor Tom Cruise. Roberts' movie has totally flattened his "Days of Thunder," which is stalled at No. 3. "Quick Change," the Bill Murray comedy, moved up four notches to No. 12. It may not be as big a rental hit as many predicted. "Arachnophobia" entered the chart at No. 28, but should bound into the Top 10 next week. Though expected to jet into the Top Five after debuting at No. 9 last week, "Air America"--Mel Gibson's latest action epic--moved up just two notches to No. 7. Not bad, but for a Gibson movie, not great.