#574 remove support for PHP 7.4.1 and newer

PHP 7.4.1 broke backward compatibility in which public typed properties no longer trigger `__get`
calls when not initialized, and must be explicitly unset instead. This is incompatible with the
`2.7.x` releases of `ocramius/proxy-manager`, so we intentionally remove compatibility to allow
downstream clients to either upgrade to `2.8.0` or `2.7.1` (if they are running *exactly* PHP
7.4.0).

Ref: https://bugs.php.net/bug.php?id=79373

Fixes #574

composer.json

@@ -19,7 +19,7 @@
         }
     ],
     "require": {
-        "php":                       "7.4.*",
+        "php":                       "7.4.0",
         "laminas/laminas-code":      "^3.4.1",
         "ocramius/package-versions": "^1.5.1",
         "webimpress/safe-writer":    "^2.0"