Adds macos build for travis

.travis.yml

@@ -1,16 +1,47 @@
 language: python
 
 python:
-  - '2.7'
+  - 2.7.13
 
-env: 
+# Use a matrix of TEST_SUITE variables to distribute tests to several machines
+# Use a global PYTEST variable to run all of the tests in a uniform fashion
+env:
   global:
     - PYTEST="bin/py.test -n 2 -vvs"
 
   matrix:
     - TEST_SUITE="$PYTEST"
     - TEST_SUITE="./etc/release/release.sh"
 
+
+# Travis does not offer OSX with arbitrary python versions (like 2.7.13 above)
+# So, you can not simply have the following section in your build matrix:
+# os:
+#   - linux
+#   - osx
+# Instead, you have to include OSX entries into the build matrix manually.
+# In particular, this means specifying the environment variables again.
+
+# The following was adapted from here:
+#   https://docs.travis-ci.com/user/multi-os/
+# Set TEST_SUITE in `env:` so that the `script:` section below works
+# Set `language: generic` to clear `language: python` from above
+# Set `python:` (to empty) to clear it from the travis-ci web interface
+# Set `osx_image: xcode7.3` to pin OSX and Python versions, see here:
+#   https://docs.travis-ci.com/user/osx-ci-environment/
+matrix:
+  include:
+    - os: osx
+      env: TEST_SUITE="$PYTEST"
+      language: generic
+      python:
+      osx_image: xcode7.3
+    - os: osx
+      env: TEST_SUITE="./etc/release/release.sh"
+      language: generic
+      python:
+      osx_image: xcode7.3
+
 install:
   - ./configure