Add RoboFile.php to root directory again.

It was previously removed in 23824dd,
but this introduced a problem where an erroring Robo command
would also print an error about a missing RoboFile.php.

This could make debugging errors with Robo commands confusing, since an unrelated error was appearing.

RoboFile.php

@@ -0,0 +1,11 @@
+<?php
+/**
+ * This is the project's configuration for Robo task runner. This doesn't actually do anything
+ * other than exist, to prevent invalid Robo commands from returning unrelated errors.
+ *
+ * @see http://robo.li/
+ */
+class RoboFile extends \Robo\Tasks
+{
+    // This space intentionally left blank.
+}