MDL-49081 tests: Removed dev option from composer

dev option is enabled by default in composer, so not required.
https://github.com/composer/composer/blob/master/CHANGELOG.md

lib/testing/lib.php

@@ -190,7 +190,7 @@ function testing_update_composer_dependencies() {
     }
 
     // Update composer dependencies.
-    passthru("php composer.phar update --dev", $code);
+    passthru("php composer.phar update", $code);
     if ($code != 0) {
         exit($code);
     }