fix Travis tests and Mailsystem - deprecation notices

Problem/Motivation

https://travis-ci.org/antistatique/drupal-backerymails/jobs/604376570

30x: There is no `base theme` property specified in the mailsystem_test_theme.info.yml file. The optionality of the `base theme` property is deprecated in drupal:8.8.0 and is removed from drupal:9.0.0. All Drupal 8 themes must add `base theme: stable` to their *.info.yml file for them to continue to work as-is in future versions of Drupal. Drupal 9 requires the `base theme` property to be specified. See https://www.drupal.org/node/3066038
    10x in UiPageTest::testCollectionEmptyPage from Drupal\Tests\backerymails\Functional
    10x in UiPageTest::testCollectionPage from Drupal\Tests\backerymails\Functional
    10x in UiPageTest::testClear from Drupal\Tests\backerymails\Functional

Proposed resolution

add the SYMFONY_DEPRECATIONS_HELPER to disabled

script:
  # Disable deprecations helper to avoid build failed caused by usage of
  # deprecations in Drupal Core.
  - export SYMFONY_DEPRECATIONS_HELPER=disabled
  - drupal-ti script