Update OSX Travis (#3083)

Author: lmazuel

.travis.yml

@@ -23,7 +23,7 @@ matrix:
       python: "pypy3.5-5.8.0"
       dist: trusty
     - os: osx
-      osx_image: xcode7.3
+      osx_image: xcode9.4
       language: generic
   allow_failures:
     - os: osx
@@ -34,7 +34,6 @@ matrix:
 # Perform the manual steps on osx to install python3 and activate venv
 before_install:
   - if [[ -n "$TRAVIS_TAG" && "$TRAVIS_PYTHON_VERSION" != "3.6" ]]; then travis_terminate 0; fi; # Deploy on 3.6
-  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update; fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew upgrade python; fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then python3 -m venv venv; fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then source venv/bin/activate; fi