Update intl + icu installation "problem" docs

[weaverryan]

See symfony/symfony#11447.

Previously, there were some issues with installation/deployment of the correct ICU data. This was mentioned here: http://symfony.com/doc/current/components/intl.html

As of the next release of 2.3, 2.5 and (of course) 2.6 (2.4 has reached EOL I believe, so it will not be fixed), this will be much easier. We should still keep some docs for older versions, but update things so that people know which versions are affected.

[javiereguiluz]

If you need it, here it is a blog post with some more information about this change: http://symfony.com/blog/new-in-symfony-2-6-farewell-to-icu-component

[weaverryan]

Yes brilliant - thanks Javier!