Add ext-intl dependency to the composer.json

If I open the user entity to edit in admin panel, I get error "Fatal error: Class 'ResourceBundle' not found ...". It looks like missed php-intl extension on my local PC. So, I included it to composer.json as required package.

I did not found any references to "SonataIntlBundle" in the source code, which depends on ext-int. Otherwise, I would include it instead of "ext-intl".