run tests against multiple Symfony versions

.travis.yml

@@ -12,9 +12,19 @@ php:
     - '7.0'
     - 'hhvm'
 
+env:
+    - SYMFONY_VERSION=locked # Use the versions that are given in composer.lock.
+    - SYMFONY_VERSION="2.8.*"
+    - SYMFONY_VERSION="3.0.*"
+    - SYMFONY_VERSION="3.1.*"
+
 before_script:
     - composer self-update --no-interaction
-    - composer install --no-interaction --prefer-source
+    - sh -c "if [ '$SYMFONY_VERSION' = 'locked' ]; then composer install --no-interaction --prefer-source; fi"
+    # Register a specific dependency versions if necessary...
+    - sh -c "if [ '$SYMFONY_VERSION' != 'locked' ]; then composer require --no-update symfony/symfony:${SYMFONY_VERSION}; fi"
+    # ... and update the dependencies accordingly, without restrictions regarding the PHP version.
+    - sh -c "if [ '$SYMFONY_VERSION' != 'locked' ]; then composer config --unset platform.php; composer update --no-interaction --prefer-source; fi"
     - mkdir -p build/logs
 
 script: