ensure all supported symfony versions are tested

.travis.yml

@@ -1,23 +1,45 @@
 language: php
 
+cache:
+  directories:
+    - $COMPOSER_CACHE_DIR
+
 php:
-    - 5.4
-    - 5.5
-    - 5.6
-    - 7.0
-    - hhvm
+  - 5.4
+  - 5.5
+  - 5.6
+  - 7.0
+  - hhvm
+
+allow_failures:
+  - php: hhvm
 
-matrix:
-    fast_finish: true
-    allow_failures:
-        - php: hhvm
 env:
-  - PREFER_LOWEST="--prefer-lowest"
-  - PREFER_LOWEST=""
+  - SYMFONY_VERSION=2.4.*
+  - SYMFONY_VERSION=2.5.*
+  - SYMFONY_VERSION=2.6.*
+  - SYMFONY_VERSION=2.7.*
+  - SYMFONY_VERSION=2.8.*
+  - SYMFONY_VERSION=3.0.*
+
+matrix:
+  #Symfony 3.0 requires PHP 5.5
+  exclude:
+    - php: 5.4
+      env: SYMFONY_VERSION=3.0.*
+  fast_finish: true
+  allow_failures:
+    - php: hhvm
+
+before_install:
+  - composer self-update
+  - composer require --no-update symfony/expression-language "$SYMFONY_VERSION"
+  - composer require --no-update --dev symfony/yaml "$SYMFONY_VERSION"
+  - composer require --no-update --dev symfony/routing "$SYMFONY_VERSION"
+  - composer require --no-update --dev symfony/dependency-injection "$SYMFONY_VERSION"
 
-before_script:
-    - composer self-update
-    - composer update --prefer-dist --no-interaction $PREFER_LOWEST
+install:
+    - composer install --no-interaction --prefer-dist
 
 script:
     - ./bin/phpunit --coverage-text