[4.1] Minor Composer Tweak

Let's make the swiftmailer version constraint consistent with the way we are requiring other stuff in laravel 4.1. The `~5.0` notation is what we are using in 4.2, and `5.*` matches how we are requiring stuff in 4.1.

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NB: For merging into 4.2, we should keep what 4.2 has, and discard the changes on the 4.1 branch.