build: hard dependencies (#6)

Author: Greg Bowler

composer.json

@@ -9,8 +9,8 @@
 		"phpgt/promise": "^2.0"
 	},
 	"require-dev": {
-		"phpunit/phpunit": "9.*",
-		"phpstan/phpstan": "^0.12"
+		"phpstan/phpstan": "v1.8.0",
+		"phpunit/phpunit": "v9.5.21"
 	},
 
 	"autoload": {