Ajaxify the update cart button

New features:

• Issue [#2910741] by subhojit777: Ajaxify the Update cart button
  • Changes made in the other test view, so that the test helper functions can be reused, without changing CSS class.
  • Fixed random failing test. The fix is done by removing the randomness in the test.
  • I was unable to find the reason why quantity didn't get updated in case of the first product variation. The test failed asserting that for the first product variation the quantity is updated. The test was passing when second product variation was randomly chosen to be updated. Even in the test screenshots it clearly showed that the quantity is updated for the first product variation.
  • Added more tests: Test whether the update cart button is indeed ajaxified. Test whether the order total is correct after quantity update.
• Known issues:
  • [#2941531]
  • [#2941532]

Test improvements:

• Issue [#2935858] by subhojit777: [Test Improvement] Move common setup tasks to the views base test class
• Issue [#2910746] by subhojit777: Fixed incorrect xpath query in the test
• TravisCI changes:
  • Notify on test failures.
  • Drupal core version lowered in TravisCI tests. This is done to suppress deprecation warnings.
  • Removed PHP 7 support. The tests which checked whether the ajax features are working, those were failing in case of Drupal 8.5 and PHP 7.
  • Drupal 8.5 support has been removed earlier. You can see it here.