Always install dev requirements when using the require command, fixes #1676

src/Composer/Command/RequireCommand.php

@@ -107,7 +107,7 @@ EOT
             ->setVerbose($input->getOption('verbose'))
             ->setPreferSource($input->getOption('prefer-source'))
             ->setPreferDist($input->getOption('prefer-dist'))
-            ->setDevMode($input->getOption('dev'))
+            ->setDevMode(true)
             ->setUpdate(true)
             ->setUpdateWhitelist(array_keys($requirements));
         ;