[travis] Add osx builds

Author: mpsonntag

.travis.yml

@@ -5,10 +5,25 @@ language: python
 
 matrix:
     include:
-        - python: "2.7"
+        - os: linux
+          python: "2.7"
           env: COVERALLS=1
-        - python: "3.4"
-        - python: "3.5"
+        - os: linux
+          python: "3.4"
+        - os: linux
+          python: "3.5"
+
+        - os: osx
+          language: generic
+          env:
+            - OSXENV=2.7
+        - os: osx
+          language: generic
+          env:
+            - OSXENV=3.5
+
+before_install:
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then bash resources/install_osx_virtualenv.sh; fi
 
 install:
   - export PYVER=${TRAVIS_PYTHON_VERSION:0:1}