fixes #329: Require ext-intl in the composer.json

[aminin]

No description provided.

[voronkovich]

@aminin, you should add it to the suggest section https://getcomposer.org/doc/04-schema.md#suggest
See #335

[javiereguiluz]

@aminin the solution that you suggest is correct. However, it's too hard to comply for some users because the intl extension is a bit tricky. That's why we've merged #344 to make this app work without that extension. Therefore, I'm closing this as "won't fix".