[CI] Use Miniconda instead of Homebrew for macOS dependencies

Installation of Homebrew packages fails when the brew version in the Travis
image gets too far out of date, but updating Homebrew as part of the CI builds
takes ages and frequently fails. The best solution seems to be to just not use
it in the first place.

.travis.yml

@@ -18,12 +18,6 @@ addons:
       - libboost-dev
       - doxygen
       - graphviz
-  homebrew:
-    packages:
-    - scons
-    - boost
-    - python
-    - libomp
   ssh_known_hosts:
     - cantera.org
 
@@ -35,8 +29,13 @@ env:
 before_script: |
   echo TRAVIS_OS_NAME: $TRAVIS_OS_NAME
   if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
-      pip3 install numpy cython  # Install numpy and Cython for Python 3
-      pip3 install ruamel.yaml
+      export CONDA_ARCH="${TRAVIS_OS_NAME}_${BUILD_ARCH}"
+      curl https://repo.continuum.io/miniconda/Miniconda3-latest-MacOSX-x86_64.sh -o miniconda.sh;
+      bash miniconda.sh -b -p $HOME/miniconda
+      source $HOME/miniconda/etc/profile.d/conda.sh && conda activate
+      conda config --set always_yes yes --set changeps1 no
+      conda install -q numpy cython scons boost ruamel_yaml
+      conda install -q -c conda-forge openmp
   else
       pip3 install --user --upgrade pip
       pip3 install --user --upgrade setuptools wheel
@@ -77,7 +76,7 @@ script: |
         echo "Skipping documentation upload from source other than Cantera/cantera:master"
       fi
   else
-      scons build -j2 python_cmd=python3 VERBOSE=y python_package=full blas_lapack_libs=lapack,blas optimize=n coverage=y
+      scons build -j2 python_cmd=python3 VERBOSE=y python_package=full blas_lapack_libs=lapack,blas optimize=n coverage=y extra_inc_dirs=$CONDA_PREFIX/include extra_lib_dirs=$CONDA_PREFIX/lib
       scons test
       scons samples
   fi