[CI] Use Travis Homebrew addon instead of manual brew commands

This should reduce the time needed to set up the build environment and avoid
build failures associated with errors while updating irrelevant homebrew
packages.

.travis.yml

@@ -18,6 +18,11 @@ addons:
       - libboost-dev
       - doxygen
       - graphviz
+  homebrew:
+    packages:
+    - scons
+    - boost
+    - python
   ssh_known_hosts:
     - cantera.org
 
@@ -29,12 +34,7 @@ env:
 before_script: |
   echo TRAVIS_OS_NAME: $TRAVIS_OS_NAME
   if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
-      brew update > /dev/null
-      brew cask uninstall oclint  # See https://github.com/travis-ci/travis-ci/issues/8826
-      brew upgrade python  # Installs Python 3
-      brew install scons  # Install SCons which *should* use Python 3
       pip3 install numpy cython  # Install numpy and Cython for Python 3
-      brew install boost
   else
       pip3 install --user --upgrade pip
       pip3 install --user --upgrade setuptools wheel