Pull MockRavenClient (which depends on Raven_Client) out of

tests/Monolog/Handler/RavenHandlerTest.php into a separate file so that
the test can pass when raven-php is installed and can be skipped when
raven-php is not installed. Addresses @Stof's comment in
https://github.com/Seldaek/monolog/pull/76#r740465

tests/Monolog/Handler/MockRavenClient.php

@@ -0,0 +1,26 @@
+<?php
+
+/*
+ * This file is part of the Monolog package.
+ *
+ * (c) Jordi Boggiano <j.boggiano@seld.be>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Monolog\Handler;
+
+use Raven_Client;
+
+class MockRavenClient extends Raven_Client
+{
+    public function capture($data, $stack)
+    {
+        $this->lastData = $data;
+        $this->lastStack = $stack;
+    }
+
+    public $lastData;
+    public $lastStack;
+}