Bump phpunit/phpunit from 5.7.21 to 7.5.20

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 5.7.21 to 7.5.20.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/7.5.20/ChangeLog-7.5.md)
- [Commits](sebastianbergmann/phpunit@5.7.21...7.5.20)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

composer.json

@@ -33,7 +33,7 @@
     "barryvdh/laravel-ide-helper": "^2.4",
     "fzaninotto/faker": "~1.4",
     "mockery/mockery": "0.9.*",
-    "phpunit/phpunit": "~5.7"
+    "phpunit/phpunit": "~7.5"
   },
   "autoload": {
     "classmap": [