Revert "Switch Travis to OSX 10.12 and Xcode 8.3.3."
Revert "Switch Travis to OSX 10.12 and Xcode 8.3.3."

This reverts commit 587e556a909fcd2e1507a9230d9cdaffa3f9394e from PR #1024.
This commit did not solve any issue at the time it was merged but made the
macOS package we produce compatible only with macOS 10.12 and later.

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