Merge branch 'feature/continuous-integration' into 'master'

Continuous Integration Updates

- Add `.travis.yml` for GitHub (first set of tests were run [here](https://travis-ci.org/PyCQA/flake8/builds/44403113))

- Update `tox.ini` to add

 - `py25` environment

 - `py{27,34}-flake8` environments

See merge request !12 (https://gitlab.com/pycqa/flake8/merge_requests/12)