Merged in feature/from-pantheon (pull request #16)

code from pantheon

* code from pantheon

wp/wp-content/plugins/wordpress-seo/src/conditionals/jetpack-conditional.php

@@ -0,0 +1,19 @@
+<?php
+
+namespace Yoast\WP\SEO\Conditionals;
+
+/**
+ * Conditional that is only met when Jetpack exists.
+ */
+class Jetpack_Conditional implements Conditional {
+
+	/**
+	 * Returns `true` when the Jetpack plugin exists on this
+	 * WordPress installation.
+	 *
+	 * @return bool `true` when the Jetpack plugin exists on this WordPress installation.
+	 */
+	public function is_met() {
+		return \class_exists( 'Jetpack' );
+	}
+}