Add allowed failure builds to build matrix

Author: wouterj

.travis.yml

@@ -25,6 +25,10 @@ matrix:
       env: SYMFONY_VERSION=2.6.*
     - php: 5.3
       env: SYMFONY_VERSION=2.3.* COMPOSER_FLAGS="--prefer-lowest"
+    - php: 5.6
+      env: SYMFONY_VERSION=2.8.*
+    - php: 5.6
+      env: SYMFONY_VERSION=3.0.*@dev
   allow_failures:
     - php: 5.6
       env: SYMFONY_VERSION=2.8.*