Update travis.yml (multiple django envs)

    - 3.4

    - 3.5

    - 3.6

env:

    - DJANGO='django>=1.8,<1.9'

    - DJANGO='django>=1.9,<1.10'

    - DJANGO='django>=1.10,<1.11'

    - DJANGO='django>=1.11,<2.0'

    - DJANGO='django>=2.0'

install:

    - travis_retry pip install -r requirements.txt