fixed Travis config

Author: craue

.travis.yml

@@ -17,6 +17,7 @@ matrix:
       dist: precise
       env: DEPS='lowest' SYMFONY_DEPRECATIONS_HELPER='2'
     - php: 5.6
+      dist: precise
       env: DEPS='unmodified'
     - php: 5.6
       env: SYMFONY_VERSION='2.7.*' PARAM_DB_DRIVER='pdo_sqlite'
@@ -25,8 +26,10 @@ matrix:
     - php: 5.6
       env: SYMFONY_VERSION='3.2.*'
     - php: 5.6
+      dist: precise
       env: SYMFONY_VERSION='3.3.*'
     - php: 5.6
+      dist: precise
       env: SYMFONY_VERSION='3.4.*' MIN_STABILITY='dev'
   allow_failures:
     - env: SYMFONY_VERSION='3.4.*' MIN_STABILITY='dev'