Travis: Use a newer macOS image

I tried some newer images, here's what I found:

Xcode 10.1:

• https://github.com/teor2345/tor/tree/macos_image_10_1
• build took longer than 50 minutes

Xcode 10.3:

• https://github.com/teor2345/tor/tree/macos_image_10_3
• build took 19 minutes, vs 9 minutes for the current image

Xcode 11.2:

• https://github.com/teor2345/tor/tree/macos_image_11_2
• 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.