Fixing dependency versions

Author: mmasiukevich

composer.json

@@ -28,14 +28,14 @@
   },
   "require": {
     "php": ">=7.3",
-    "amphp/amp": "^2",
-    "amphp/file": "^0.3"
+    "amphp/amp": "v2.1.*",
+    "amphp/file": "v0.3.*"
   },
   "require-dev": {
-    "php-service-bus/code-style-config": "^v0.1",
-    "phpunit/phpunit": "^8",
-    "vimeo/psalm": "^v3",
-    "phpstan/phpstan": "0.11.*"
+    "php-service-bus/code-style-config": "v0.1.*",
+    "phpunit/phpunit": "v8.3.*",
+    "vimeo/psalm": "v3.4.*",
+    "phpstan/phpstan": "v0.11.*"
   },
   "prefer-stable": true,
   "minimum-stability": "dev",