Fix sudden HomeBrew 'python not installed' error

Author: YannickJadoul

.travis.yml

@@ -243,7 +243,7 @@ before_install:
       PY_CMD=python$PYTHON
       if [ "$TRAVIS_OS_NAME" = "osx" ]; then
         if [ "$PY" = "3" ]; then
-          brew update && brew unlink python@2 && brew upgrade python
+          brew update && brew unlink python@2 && (brew upgrade python || brew install python)
         else
           curl -fsSL https://bootstrap.pypa.io/get-pip.py | $PY_CMD - --user
         fi