Updates and fixes

[lmakarov]

• Fixed outdated failing tests for https://getbootstrap.com/
• Updated behat dependencies
• Pined selenium images and switched to "debug" versions to make debugging with VNC available out of the box:
  • selenium/standalone-chrome-debug:3.141.59
  • selenium/standalone-firefox-debug:3.141.59
• CI updates
  • Switched base image to alpine:3.12
  • Switched Travis CI to Ubuntu 20.04 LTS (focal)
  • Run behat tests in CI
  • Bumped docker-compose.yaml version
• Give selenium some time to initialize
  • Fixes "Element not found with xpath, //html" error in tests (fixes #15 , fixes #7, closes #10 )

[lmakarov]

The example setup and tests from the example folder pass in CI: https://travis-ci.com/github/docksal/behat/builds/207409190