Implicitly create composer.json in require command

This allows shorteninig install instructions if you do not want to use init
(because it is interactive) and you do not want to use create-project (there
is no skeleton, or you do not want to use a skeleton).

src/Composer/Command/RequireCommand.php

@@ -54,8 +54,8 @@ EOT
     {
         $file = Factory::getComposerFile();
 
-        if (!file_exists($file)) {
-            $output->writeln('<error>'.$file.' not found.</error>');
+        if (!file_exists($file) && !file_put_contents($file, "{}\n")) {
+            $output->writeln('<error>'.$file.' could not be created.</error>');
 
             return 1;
         }