Tidy up integrations, use contracts

app/Integrations/Contracts/Credits.php

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+<?php
+
+/*
+ * This file is part of Cachet.
+ *
+ * (c) Alt Three Services Limited
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace CachetHQ\Cachet\Integrations\Contracts;
+
+/**
+ * This is the credits interface.
+ *
+ * @author James Brooks <james@alt-three.com>
+ */
+interface Credits
+{
+    /**
+     * Returns the latest credits.
+     *
+     * @return array|null
+     */
+    public function latest();
+}

app/Integrations/Contracts/Feed.php

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+<?php
+
+/*
+ * This file is part of Cachet.
+ *
+ * (c) Alt Three Services Limited
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace CachetHQ\Cachet\Integrations\Contracts;
+
+/**
+ * This is the feed interface.
+ *
+ * @author James Brooks <james@alt-three.com>
+ */
+interface Feed
+{
+    /**
+     * Returns the latest entries.
+     *
+     * @return array|null
+     */
+    public function latest();
+}

app/Integrations/Contracts/Releases.php

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+<?php
+
+/*
+ * This file is part of Cachet.
+ *
+ * (c) Alt Three Services Limited
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace CachetHQ\Cachet\Integrations\Contracts;
+
+/**
+ * This is the releases interface.
+ *
+ * @author James Brooks <james@alt-three.com>
+ */
+interface Releases
+{
+    /**
+     * Returns the latest release.
+     *
+     * @return string
+     */
+    public function latest();
+}