Stick with Drush 8 for now.

.travis.yml

@@ -39,7 +39,7 @@ install:
   - test ${DRUPAL_VERSION} -ne 8 || (cp doc/_static/composer.json.d8 ./composer.json && composer require --prefer-source drush-ops/behat-drush-endpoint drupal/drupal-driver:dev-master)
   - composer install
   # Drush version must vary depending on environment and version of core.
-  - test ${TRAVIS_PHP_VERSION} == "5.3" && composer global require drush/drush:~6.0 || composer global require drush/drush:dev-master
+  - test ${TRAVIS_PHP_VERSION} == "5.3" && composer global require drush/drush:~6.0 || composer global require drush/drush:~8.0
   # Install the Behat Drush Endpoint for Drupal 7 tests.
   - test ${DRUPAL_VERSION} -ne 7 || (git clone https://github.com/drush-ops/behat-drush-endpoint.git drush/behat-drush-endpoint && (cd drush/behat-drush-endpoint && composer install))
   # PHP 5.3 requires the cgi extension for runserver.