update travis config to use debian python and install mock

.travis.yml

@@ -1,10 +1,12 @@
 language: python
 python:
-  - "2.7"
+  - "2.7_with_system_site_packages"
   - "3.3"
 # command to install dependencies
 install:
-  - pip install argparse catkin-pkg empy nose
+  # fetch "normal" debian python for 3.3
+  - if [ "$TRAVIS_PYTHON_VERSION" == "3.3" ]; then sudo add-apt-repository -y ppa:fkrull/deadsnakes && sudo apt-get -y update && sudo apt-get install python3.3 python3.3-dev && virtualenv -p /usr/bin/python3.3 ~/virtualenvs/3.3_debian && source ~/virtualenvs/3.3_debian/bin/activate; fi
+  - pip install argparse catkin-pkg empy mock nose
 # command to run tests
 script:
   - nosetests test/unit_tests test/local_tests