Configured the Travis matrix so tests are run with Symfony 2.8, 3.3, …

…3.4 and 4.x

.travis.yml

@@ -8,15 +8,26 @@ cache:
   directories:
     - $HOME/.composer/cache/files
 
-php:
-  - 5.5
-  - 5.6
-  - 7.0
-  - 7.1
+matrix:
+  include:
+    - php: 5.5
+    - php: 5.6
+      env: SYMFONY_VERSION=2.8
+    - php: 5.6
+      env: SYMFONY_VERSION=3.3
+    - php: 5.6
+    - php: 7.0
+    - php: 7.1
 
 before_script:
-  - composer self-update
-  - COMPOSER_ROOT_VERSION=dev-master composer install
+  - |
+    if [[ $SYMFONY_VERSION = 2.8 ]]; then
+      composer remove --dev --no-update symfony/web-server-bundle
+      composer require --dev --no-update 'symfony/console:2.8.*' 'symfony/twig-bridge:2.8.*' 'symfony/monolog-bridge:2.8.*' 'symfony/yaml:2.8.*'
+    fi
+    if [[ $SYMFONY_VERSION = 3.3 ]]; then
+      composer require --dev --no-update 'symfony/console:3.3.*' 'symfony/twig-bridge:3.3.*' 'symfony/monolog-bridge:3.3.*' 'symfony/yaml:3.3.*' 'symfony/web-server-bundle:3.3.*'
+    fi
+  - COMPOSER_ROOT_VERSION=dev-master composer update --no-suggest
 
-script:
-  - ./vendor/bin/simple-phpunit
+script: ./vendor/bin/simple-phpunit