⬆️ 🔨 Upgrade PHP Markdown library.

The current version throws deprecated warning with PHP7.1 and PHPUnit.
Upgrade the Markdown library to the current PHP Markdown Lib 1.7.0.
Used composer to manage this library.

vendor/composer/autoload_namespaces.php

@@ -6,4 +6,5 @@ $vendorDir = dirname(dirname(__FILE__));
 $baseDir = dirname($vendorDir);
 
 return array(
+    'Michelf' => array($vendorDir . '/michelf/php-markdown'),
 );