use deprecation helper for twig warnings too

Author: dbu

.travis.yml

@@ -26,7 +26,7 @@ matrix:
     - php: 5.6
       env: SYMFONY_VERSION=3.0.*
     - php: 5.3
-      env: SYMFONY_VERSION=2.3.* COMPOSER_FLAGS="--prefer-lowest"
+      env: SYMFONY_VERSION=2.3.* COMPOSER_FLAGS="--prefer-lowest" SYMFONY_DEPRECATIONS_HELPER=weak
   allow_failures:
     - php: nightly
     - env: SYMFONY_VERSION=2.8.*