Update phan/phan requirement from ^1.1 to ^1.1 || ^2.0

Updates the requirements on [phan/phan](https://github.com/phan/phan) to permit the latest version.
- [Release notes](https://github.com/phan/phan/releases)
- [Changelog](https://github.com/phan/phan/blob/master/NEWS.md)
- [Commits](phan/phan@1.1.0...2.1.0)

composer.json

@@ -19,7 +19,7 @@
         "infection/infection": ">=0.10.5",
         "league/pipeline": "^1.0 || ^0.3",
         "mockery/mockery": "^1.0",
-        "phan/phan": "^1.1",
+        "phan/phan": "^1.1 || ^2.0",
         "php-coveralls/php-coveralls": "^2.0",
         "phpstan/phpstan": ">=0.9",
         "phpunit/phpunit": ">=6 <8",