Added php-json as required PHP module

Without php-json the installation stops with a white screen and the following error:
> 09-Nov-2019 14:05:46 UTC] PHP Fatal error:  Uncaught Error: Call to undefined function Shaarli\Config\json_encode() in /var/www/html/shaarli/application/config/ConfigJson.php:48
> Stack trace:
> #0 /var/www/html/shaarli/application/config/ConfigManager.php(239): Shaarli\Config\ConfigJson->write('data/config.jso...', Array)
> #1 /var/www/html/shaarli/index.php(1835): Shaarli\Config\ConfigManager->write(false)
> #2 /var/www/html/shaarli/index.php(178): install(Object(Shaarli\Config\ConfigManager), Object(Shaarli\Security\SessionManager), Object(Shaarli\Security\LoginManager))
> #3 {main}
>   thrown in /var/www/html/shaarli/application/config/ConfigJson.php on line 48

Tested with Shaarli 0.10.4 on CentOS 8 with Httpd 2.4.37 and PHP 7.2.11.