update psalm/plugin-phpunit requirement from ^0.16.1 to ^0.18.4 (via allure-framework#83)

Author: dependabot[bot]

composer.json

@@ -35,7 +35,7 @@
   },
   "require-dev": {
     "brianium/paratest": "^6.4.1",
-    "psalm/plugin-phpunit": "^0.16.1",
+    "psalm/plugin-phpunit": "^0.18.4",
     "squizlabs/php_codesniffer": "^3.6.2",
     "vimeo/psalm": "^4.16.1"
   },