Upgrade brew's Python to avoid linking errors

[iriberri]

See: Homebrew/homebrew-core#26358

After Python's formula was recently updated, macOS builds have been failing with the error:

Error: The `brew link` step did not complete successfully
The formula built, but is not symlinked into /usr/local
Could not symlink bin/2to3-2
Target /usr/local/bin/2to3-2 is a symlink belonging to python. 

For example: https://travis-ci.org/varnishcache/varnish-cache/jobs/370937346

Until Python is upgraded in the Travis base image, the workaround would be to manually do so during the steps of the build.

Hope this helps!

[fgsch]

It does, thank you!

[fgsch]

Merged, thanks for your help.