Declare dependency on ext/intl

Composer's error messages are confusing if the `intl` extension is not present on a system, and upcoming extension installers (think Pickle integration etc) or other means (my use case: auto-enable intl on Heroku if a dependency needs it) can enable the extension without having to alias "lib-icu" to "ext-intl" somehow.

composer.json

@@ -18,6 +18,7 @@
     "require": {
         "php": ">=5.3.3",
         "lib-ICU": ">=4.4",
+        "ext-intl": "*",
         "symfony/intl": "~2.3"
     },
     "autoload": {