Update all dependencies

[gin0115]

The core dependencies used for this library need to be updated. This is mainly for the dev dependencies.

{    "require-dev": {
        "phpunit/phpunit": "^7.0",
        "roots/wordpress": "^5.8",
        "wp-phpunit/wp-phpunit": "^5.8",
        "symfony/var-dumper": "4.*",
        "phpstan/phpstan": "^1.0",
        "szepeviktor/phpstan-wordpress": "^1.0",
        "php-stubs/wordpress-stubs": "^5.8.0",
        "dealerdirect/phpcodesniffer-composer-installer": "*",
        "wp-coding-standards/wpcs": "*",
        "object-calisthenics/phpcs-calisthenics-rules": "*",
        "yoast/phpunit-polyfills": "^0.2.0 || ^1.0.0",
        "gin0115/wpunit-helpers": "~1.0.0"
    }
}