CI: use installed homebrew Python on travis OS X

Speed up OS X homebrew build on travis.

Author: pavoljuhas

.travis.yml

@@ -76,7 +76,6 @@ before_install:
 
   - $NOBREW || test "${TRAVIS_OS_NAME}" = "osx" || exit $?
   - $NOBREW || brew update
-  - $NOBREW || brew upgrade python
   - $NOBREW || brew install gcc || brew link --overwrite gcc
   - $NOBREW || brew install boost-python3
   - $NOBREW || brew install gsl