NEW YORK — One of the oldest trading strategies in the book is showing a yellow flag for stock investors, but analysts say it's not quite time to sell yet. The Dow Jones industrial average closed at an all-time high last month, extending a bull market that stretches back more than six years. However, another gauge that tracks airlines, railroad and freight companies — the Dow Jones transportation average — has gone the other way, slumping to a seven-month …
