Add support for Swiftmailer 6 (#37)

Author: fabpot

composer.json

@@ -11,7 +11,7 @@
     ],
     "require" : {
         "php": ">=7.0",
-        "swiftmailer/swiftmailer": "^5.1",
+        "swiftmailer/swiftmailer": "^5.1 || ^6.0",
         "tijsverkoyen/css-to-inline-styles": "^2.1"
     },
     "require-dev" : {

composer.lock

@@ -35,7 +35,7 @@ public function setUp()
         $this->emailRaw = file_get_contents($dir.'emailRaw.html');
         $this->emailConverted = file_get_contents($dir.'emailConverted.html');
 
-        $this->mailer = Swift_Mailer::newInstance(Swift_NullTransport::newInstance());
+        $this->mailer = new Swift_Mailer(new Swift_NullTransport);
         $this->mailer->registerPLugin(new CssInlinerPlugin());
     }
 
@@ -44,7 +44,7 @@ public function setUp()
      */
     private function createMessage()
     {
-        $message = Swift_Message::newInstance();
+        $message = new Swift_Message();
 
         $message->setFrom('test@example.com');
         $message->setTo('test2@example.com');
@@ -109,7 +109,7 @@ public function testInjectedConverterIsUsedInsteadOfDefault()
         $message = $this->createMessage();
         $message->setContentType('text/html');
 
-        $mailer = Swift_Mailer::newInstance(Swift_NullTransport::newInstance());
+        $mailer = new Swift_Mailer(new Swift_NullTransport);
         $mailer->registerPlugin(new CssInlinerPlugin($converterStub));
         $mailer->send($message);
     }