composer.json: fixed minimum requirements

Session use `Nette\Utils\Callback::invokeSafe` which is available in nette/utils since 2.2.2. How did I figure it out?

~~~sh
composer update --prefer-lowest && ./vendor/bin/tester -c ./tests/php-win.ini ./tests/
~~~

The question is – should we check this [automatically](https://github.com/symfony/symfony/commit/ef83a6cd6dae10e7af24329e547c1e3f117ede22)?

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Note: this is available in Composer since [since Sunday](https://github.com/composer/composer/commit/d621c51b2ce1c2e1781ffb44b15c2a76a5ee0069).