Update Python 2.7.15 and Django 1.11.14

What

• Update Python2.7.9 to Python2.7.15.
• Update Django1.8.1 to Django1.11.14.
• Update Third-party libraries.
• Fix .gitlab-ci.yml
  • Use PostgreSQL with the Docker executor
    • FYI: https://docs.gitlab.com/ce/ci/services/postgres.html
  • Add dpl option --skip-cleanup.
    • FYI: https://gitlab.com/gitlab-org/gitlab-ce/issues/33115
• Update Procfile.
  • Specifying release phase tasks.
    • FYI: https://devcenter.heroku.com/articles/release-phase#specifying-release-phase-tasks