Update sensiolabs/security-checker requirement from ^5.0 to ^6.0 (#11)

Updates the requirements on [sensiolabs/security-checker](https://github.com/sensiolabs/security-checker) to permit the latest version.
- [Release notes](https://github.com/sensiolabs/security-checker/releases)
- [Commits](sensiolabs/security-checker@v5.0.0...v6.0.0)

composer.json

@@ -38,7 +38,7 @@
         "php-coveralls/php-coveralls": "^2.1",
         "phpstan/phpstan": "^0.11",
         "phpunit/phpunit": "^7.5|^8.0",
-        "sensiolabs/security-checker": "^5.0",
+        "sensiolabs/security-checker": "^6.0",
         "symfony/phpunit-bridge": "^3.0|^4.0"
     },