proper version for http-factory-tests

Author: codemasher

composer.json

@@ -44,7 +44,7 @@
 	"require-dev": {
 		"phpunit/phpunit": "^8.4",
 		"guzzlehttp/psr7": "^1.6",
-		"http-interop/http-factory-tests": "dev-master"
+		"http-interop/http-factory-tests": "^0.6"
 	},
 	"autoload": {
 		"files": [