Use language: generic for Mac builds on Travis

https://github.com/travis-ci/travis-ci/issues/2312

.travis.yml

@@ -1,25 +1,38 @@
-language: python
+language: generic
 
 matrix:
   include:
-    # Linux
+    # Linux Python 2
     - sudo: required
+      language: python
       services:
         - docker
       env:
         - "PYTHON=python2"
+
+    # Linux Python 3
     - sudo: required
+      language: python
       services:
         - docker
       env:
         - "PYTHON=python3"
-    # macOS
+      before_install:
+        - sudo apt-get -qq update
+        - sudo apt-get install -y python3-pip
+
+    # macOS Python 2
     - os: osx
       env:
         - "PYTHON=python2"
+
+    # macOS Python 3
     - os: osx
       env:
         - "PYTHON=python3"
+      before_install:
+        - brew update
+        - brew install python3
 
 script:
   - |
@@ -27,10 +40,6 @@ script:
       $PYTHON -m pip install -r requirements-dev.txt
       $PYTHON ./bin/run_tests.py
     else
-      if [[ "$PYTHON" == python3 ]]; then
-        sudo apt-get -qq update
-        sudo apt-get install -y python3-pip
-      fi
       # linux test requires root to clean up the wheelhouse (docker runs as root)
       sudo $PYTHON -m pip install -r requirements-dev.txt
       sudo $PYTHON ./bin/run_tests.py