Update .travis.yml

Author: frmatias

.travis.yml

@@ -12,4 +12,5 @@ install:
   - if [ "$TRAVIS_PYTHON_VERSION" == "3.2" ]; then travis_retry pip install coverage==3.7.1; fi
   - if [ "$TRAVIS_PYTHON_VERSION" != "3.2" ]; then travis_retry pip install coverage; fi
   - "pip install pytest pytest-cov"
-script: py.test --doctest-modules --cov ./
+script: py.test --doctest-modules --cov ./
+os: osx