Add composer hints to let 'composer install' run on module.

Scrutinizer will download the module and run 'composer install' to check
the module's code. Without these changes, it is looking for a stable version
of 3.2, which doesn't currently exist.

composer.json

@@ -20,5 +20,7 @@
 		"branch-alias": {
 			"dev-master": "1.0.x-dev"
 		}
-	}
+	},
+	"prefer-stable": true,
+	"minimum-stability": "dev"
 }