Update .travis.yml to use python3 for flake8, check_rst and coverage

14 jobs for fix-travis
Status Job ID Name Coverage
  Test
canceled #3509
check_rst

canceled #3521
coverage

canceled #3508
flake8

canceled #3513
py27-django110

canceled #3510
py27-django17

canceled #3511
py27-django18

canceled #3512
py27-django19

canceled #3517
py34-django110

canceled #3514
py34-django17

canceled #3515
py34-django18

canceled #3516
py34-django19

canceled #3520
py35-django110

canceled #3518
py35-django18

canceled #3519
py35-django19