CNBC's Jim Cramer on Wednesday said he's becoming more careful when it comes to highly-speculative stocks, saying it's time for investors to worry about market froth."I can no longer be so sanguine about these super speculative stocks that keep roaring around here. I can no longer just say it's going higher," he said. "I will be more circumspect in the future."Cramer said he's felt speculative stocks would largely keep roaring because there is such high-demand for them. But he said it's now worthwhile to "f ...