Add missing cython dependency for setup.py, so sdist can work. This f…

…ixes #59

.travis.yml

@@ -40,6 +40,7 @@ script:
       $PYTHON -m pip install $PIP_INSTALL_OPTIONS twine
       if [[ $TRAVIS_OS_NAME == osx ]]; then
         $PYTHON -m pip install $PIP_INSTALL_OPTIONS setuptools
+        $PYTHON -m pip install $PIP_INSTALL_OPTIONS cython
         $PYTHON setup.py sdist
         $PYTHON -m twine upload dist/*.zip
       fi