Revert "gmp: Use version 6 as the default"
Revert "gmp: Use version 6 as the default"

This reverts commit 370fc79b948438694dc4767570afe43c5b1fb7c4, because it causes
trouble on both Linux and Darwin, and we probably shouldn't have to worry about
that kind of stuff right now with the 15.06 release being late'ish already.

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