Update composer.json

Author: GrahamCampbell

composer.json

@@ -34,7 +34,7 @@
 		"guzzlehttp/psr7": "^1.2",
 		"php-http/guzzle6-adapter": "^1.0 || ^2.0",
 		"php-http/mock-client": "^1.2",
-		"phpunit/phpunit": "^5.7.27 || ^6.5"
+		"phpunit/phpunit": "^5.7.27 || ^7.5.15"
 	},
 	"autoload": {
 		"psr-4": { "Gitlab\\": "lib/Gitlab/" }