[6.0] f7831b621 Upgrade brew's Python to avoid linking errors

[Dridi Boukelmoune]

commit f7831b62113e754d96c08b1ddd1be2a43a10a005
Author: Carla <iriberrig at gmail.com>
Date:   Wed Apr 25 12:35:55 2018 +0200

    Upgrade brew's Python to avoid linking errors
    
    See: https://github.com/Homebrew/homebrew-core/issues/26358

diff --git a/.travis.yml b/.travis.yml
index 76a6f705a..2a8408884 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -34,6 +34,7 @@ notifications:
 before_install:
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
       brew update;
+      brew upgrade python;
       brew install docutils sphinx-doc nghttp2;
     fi
   - if [[ -n "$CLANG" ]]; then