update dev-environment to use more recent versions, requries php-7.3 now (dev-only)

Signed-off-by: Michael Kaufmann <d00p@froxlor.org>

tests/Froxlor/SettingsTest.php

@@ -8,7 +8,8 @@ use PHPUnit\Framework\TestCase;
 class SettingsTest extends TestCase
 {
 
-	protected function setUp() {
+	protected function setUp(): void
+	{
 		// start fresh
 		\Froxlor\Settings::Stash();
 	}