Fix setDefaultValueCallback

Travis build (https://travis-ci.org/md-systems/redirect/jobs/36773521) failing with below trace:

    Enabled modules: redirect, link, field, system, user.
Exception Uncaught e BaseFieldDefiniti  415 Drupal\Core\Field\BaseFieldDefiniti
    InvalidArgumentException: Default value callback must be a string, like
    &quot;function_name&quot; or &quot;ClassName::methodName&quot; in
    Drupal\Core\Field\BaseFieldDefinition-&gt;setDefaultValueCallback() (line
    415 of
    /home/travis/build/md-systems/drupal/core/lib/Drupal/Core/Field/BaseFieldDefinition.php).
    Drupal\Core\Field\BaseFieldDefinition->setDefaultValueCallback(Array)
    Drupal\redirect\Entity\Redirect::baseFieldDefinitions(Object)
    Drupal\Core\Entity\EntityManager->buildBaseFieldDefinitions('redirect')
    Drupal\Core\Entity\EntityManager->getBaseFieldDefinitions('redirect')
    Drupal\Core\Entity\EntityManager->getFieldStorageDefinitions('redirect')
    Drupal\Core\Entity\Sql\SqlContentEntityStorageSchema->__construct(Object,
    Object, Object, Object)
    Drupal\Core\Entity\Sql\SqlContentEntityStorage->getStorageSchema()
    Drupal\Core\Entity\Sql\SqlContentEntityStorage->onEntityTypeCreate(Object)
    Drupal\Core\Entity\EntityManager->onEntityTypeCreate(Object)
    Drupal\simpletest\KernelTestBase->installEntitySchema('redirect')
    Drupal\redirect\Tests\RedirectAPITest->setUp()
    Drupal\simpletest\TestBase->run()
    simpletest_script_run_one_test('1', 'Drupal\redirect\Tests\RedirectAPITest')



This is an impact of https://www.drupal.org/node/2346911