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.
bug #11 Declare dependency on ext/intl (dzuelke)

This PR was merged into the 1.2.x branch.

Discussion
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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.

Commits
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36f7e55 Declare dependency on ext/intl