Bump minimum alpha

Author: driesvints

composer.json

@@ -83,7 +83,7 @@
         "filp/whoops": "^2.4",
         "guzzlehttp/guzzle": "^6.5.5|^7.0.1",
         "league/flysystem-aws-s3-v3": "^2.0",
-        "league/flysystem-ftp": "^2.0",
+        "league/flysystem-ftp": "^2.0.0-alpha.4|^2.0",
         "league/flysystem-sftp": "^2.0",
         "mockery/mockery": "^1.3.1",
         "orchestra/testbench-core": "^6.0",