Improve travis configuration

.travis.yml

@@ -1,21 +1,39 @@
 language: php
 
 php:
-    - 5.3
-    - 5.4
-    - 5.5
-    - 5.6
-    - hhvm
-
+  - 5.3
+  - 5.4
+  - 5.5
+  - 5.6
+  - hhvm
+  - nightly
+
+env:
+  - SYMFONY_VERSION=2.6.*
+
 matrix:
-    allow_failures:
-        - php: hhvm
+  include:
+    - php: 5.3.3
+      env: SYMFONY_VERSION=2.6.* COMPOSER_FLAGS="--prefer-lowest"
+    - php: 5.6
+      env: SYMFONY_VERSION=2.6.*
+    - php: 5.6
+      env: SYMFONY_VERSION=2.7.*@dev
+    - php: 5.6
+      env: SYMFONY_VERSION=2.8.*@dev
+  allow_failures:
+    - php: hhvm
+    - env: SYMFONY_VERSION=2.7.*@dev
+    - env: SYMFONY_VERSION=2.8.*@dev
+    - php: nightly
 
 services: mongodb
 
 before_script:
-    - echo "extension=mongo.so" >> `php --ini | grep "Loaded Configuration" | sed -e "s|.*:\s*||"`
-    - composer update --dev
+  - echo "extension=mongo.so" >> `php --ini | grep "Loaded Configuration" | sed -e "s|.*:\s*||"`
+  - composer selfupdate
+  - composer require symfony/symfony:${SYMFONY_VERSION} --no-update
+  - composer update --prefer-source $COMPOSER_FLAGS
 
 notifications:
   email: travis-ci@generation-multiple.com