Travis: Use a newer macOS image
I tried some newer images, here's what I found:
- build took longer than 50 minutes
- build took 19 minutes, vs 9 minutes for the current image
- build took 12 minutes, vs 9 minutes for the current image
Let's revisit in a few months.
We could try older images, and see if they are faster. But I don't think macOS is limiting our builds right now.
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