Update phpunit

Author: snapshotpl

Diff for: composer.json

@@ -22,7 +22,7 @@
         "psr/http-factory-implementation": "^1.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^7.5.13",
+        "phpunit/phpunit": "^7.5.20",
         "mikey179/vfsStream": "^1.6.4",
         "slim/slim": "^3.0",
         "zendframework/zend-expressive": "^3.0",