Update dev requirements and test suite for PHP 7+ only

tests/Encoder/TimestampFirstCombCodecTest.php

@@ -20,7 +20,7 @@ class TimestampFirstCombCodecTest extends TestCase
      */
     private $builderMock;
 
-    protected function setUp()
+    protected function setUp(): void
     {
         $this->builderMock = $this->getMockBuilder(UuidBuilderInterface::class)->getMock();
         $this->codec = new TimestampFirstCombCodec($this->builderMock);