Improving OSX times by reducing dependencies and caching interpreters

.travis.yml

@@ -50,18 +50,23 @@ stage_osx: &stage_osx
   os: osx
   osx_image: xcode9.2
   language: generic
-  addons:
-    homebrew:
-      packages:
-      - pyenv
-      update: true
+  cache:
+    pip: true
+    directories:
+      - ~/python-interpreters/
   before_install:
     # Travis does not provide support for Python 3 under osx - it needs to be
     # installed manually.
     |
     if [ ${TRAVIS_OS_NAME} = "osx" ]; then
-      pyenv install 3.6.5
-      virtualenv --python ~/.pyenv/versions/3.6.5/bin/python venv
+      if [[ ! -d ~/python-interpreters/$PYTHON_VERSION ]]; then
+        git clone git://github.com/pyenv/pyenv.git
+        cd pyenv/plugins/python-build
+        ./install.sh
+        cd ../../..
+        python-build $PYTHON_VERSION ~/python-interpreters/$PYTHON_VERSION
+      fi
+      virtualenv --python ~/python-interpreters/$PYTHON_VERSION/bin/python venv
       source venv/bin/activate
     fi
 
@@ -110,20 +115,26 @@ jobs:
       python: 3.7
       sudo: true
 
+    # OSX, Python 3.5.6 (via pyenv)
+    - stage: test
+      <<: *stage_osx
+      env:
+        - MPLBACKEND=ps
+        - PYTHON_VERSION=3.5.6
+
     # OSX, Python 3.6.5 (via pyenv)
     - stage: test
       <<: *stage_osx
+      env:
+        - MPLBACKEND=ps
+        - PYTHON_VERSION=3.6.5
 
-    # OSX, Python 3.7 (via pyenv)
+    # OSX, Python 3.7.2 (via pyenv)
     - stage: test
       <<: *stage_osx
-      before_install:
-        |
-        if [ ${TRAVIS_OS_NAME} = "osx" ]; then
-          pyenv install 3.7.0
-          virtualenv --python ~/.pyenv/versions/3.7.0/bin/python venv
-          source venv/bin/activate
-        fi
+      env:
+        - MPLBACKEND=ps
+        - PYTHON_VERSION=3.7.2
 
     # "ibmq" stage
     ###########################################################################