Eliminate Python 2.6 which is EOL and add flake8 (#593)

* Eliminate Python 2.6 which is EOL and add flake8

* Remove Python 2.6 because it [went EOL](https://devguide.python.org/#branchstatus) five years ago.
* Add [flake8](http://flake8.pycqa.org) to find Python syntax errors and undefined names.

* Keep Python 2.6 for now

* No flake8 on Python 2.6

.travis.yml

@@ -9,6 +9,11 @@ install:
   - pip install PyYAML argparse rospkg vcstools catkin_pkg python-dateutil rosdistro
   - python setup.py build develop
   - pip install nose coverage flake8
+before_script:
+  # stop the build if there are Python syntax errors or undefined names
+  - if [ "${TRAVIS_PYTHON_VERSION}" != "2.6" ] ; then flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics; fi
+  # exit-zero treats all errors as warnings.  The GitHub editor is 127 chars wide
+  - if [ "${TRAVIS_PYTHON_VERSION}" != "2.6" ] ; then flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics; fi
 # command to run tests
 script:
   - nosetests --with-coverage --cover-package=rosdep2 --with-xunit test