John Spence: Investors are buying ETFs that follow specialized low-volatility benchmarks at a much faster pace so far this year as they hunt for funds that provide exposure to stocks but with less risk.   "'Min Vol’ strategies are ‘the new black’ for many in the industry—products which hold their relevance through a market cycle ” ConvergEx Group market strategists said in a note Tuesday. “While there are many quantitative methods to dampen portfolio volatility the two most widely used among ETF sponsors are dividend-focused funds and those products that choose investments based on historical price volatility.” Dividend ETFs ...