Suggest, not require phan

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

composer.json

@@ -25,7 +25,9 @@
 	},
 	"require-dev": {
 		"christophwurst/nextcloud": "^12.0.0",
-		"phpunit/phpunit": "^5.7",
+		"phpunit/phpunit": "^5.7"
+	},
+	"suggest": {
 		"phan/phan": "^0.10.0"
 	}
 }