Added upgrade.php file to keep Moodle's plugin DB happy

db/upgrade.php

@@ -0,0 +1,35 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Customcert module upgrade code.
+ *
+ * @package    mod_customcert
+ * @copyright  2016 Mark Nelson <markn@moodle.com>
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+defined('MOODLE_INTERNAL') || die;
+
+/**
+ * Customcert module upgrade code.
+ *
+ * @param int $oldversion the version we are upgrading from
+ * @return bool always true
+ */
+function xmldb_customcert_upgrade($oldversion) {
+    return true;
+}