Fix minimum requirement for amphp/amp

This ensures that elements are ordered correctly.

composer.json

@@ -25,7 +25,7 @@
     "require": {
         "php": ">=7",
         "amphp/parallel": "^0.1.8 || ^0.2",
-        "amphp/amp": "^2",
+        "amphp/amp": "^2.0.3",
         "opis/closure": "^3.0.7"
     },
     "require-dev": {

test/MapTest.php

@@ -14,6 +14,14 @@ class MapTest extends TestCase {
         })));
     }
 
+    public function testCorrectOutputOrder() {
+        $this->assertSame([0, 1, 0], wait(parallelMap([0, 1, 0], function ($input) {
+            sleep($input);
+
+            return $input;
+        })));
+    }
+
     public function testException() {
         $this->expectException(MultiReasonException::class);