Fix .travis.yml for updated OSX image
Fix .travis.yml for updated OSX image

Since [1], OSX builds on travis fail with:

Error: Refusing to uninstall /usr/local/Cellar/python/3.6.5_1
because it is required by gdal, numpy, postgis, which are currently installed.

Revise #3163

[1] https://blog.travis-ci.com/2018-07-19-xcode9-4-default-announce

mesonbuild/meson,pexip/meson,mesonbuild/meson,QuLogic/meson,pexip/meson,MathieuDuponchelle/meson,jeandet/meson,thiblahute/meson,jpakkane/meson,QuLogic/meson,becm/meson,becm/meson,thiblahute/meson,MathieuDuponchelle/meson,mesonbuild/meson,thiblahute/meson,MathieuDuponchelle/meson,jpakkane/meson,thiblahute/meson,jpakkane/meson,jeandet/meson,pexip/meson,pexip/meson,thiblahute/meson,mesonbuild/meson,becm/meson,QuLogic/meson,QuLogic/meson,thiblahute/meson,mesonbuild/meson,QuLogic/meson,jeandet/meson,jpakkane/meson,MathieuDuponchelle/meson,becm/meson,jeandet/meson,QuLogic/meson,becm/meson,becm/meson,QuLogic/meson,jeandet/meson,MathieuDuponchelle/meson,mesonbuild/meson,QuLogic/meson,pexip/meson,jpakkane/meson,thiblahute/meson,jpakkane/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,becm/meson,pexip/meson,jeandet/meson,MathieuDuponchelle/meson,QuLogic/meson,jeandet/meson,becm/meson,jpakkane/meson,thiblahute/meson,thiblahute/meson,mesonbuild/meson,mesonbuild/meson,jpakkane/meson,jeandet/meson,pexip/meson,pexip/meson,pexip/meson,jeandet/meson,mesonbuild/meson,becm/meson,mesonbuild/meson,pexip/meson,MathieuDuponchelle/meson,jpakkane/meson,becm/meson