add symfony to travis build

.travis.yml

@@ -8,11 +8,17 @@ matrix:
         - php: 5.4
           env: DEPENDENCIES=low
         - php: 5.5
+          env: SYMFONY_VERSION=^2.0
         - php: 5.6
+          env: SYMFONY_VERSION=^2.0
         - php: 7.0
+          env: SYMFONY_VERSION=^3.0
         - php: 7.1
+          env: SYMFONY_VERSION=^4.0
         - php: 7.2
+          env: SYMFONY_VERSION=^4.0
         - php: nightly
+          env: SYMFONY_VERSION=^4.0
     allow_failures:
       - php: nightly
     fast_finish: true
@@ -25,6 +31,7 @@ env:
 
 before_script:
     - composer self-update
+    - if [ "$SYMFONY_VERSION" != "" ]; then composer require "symfony/symfony:${SYMFONY_VERSION}" --no-update; fi;
     - if [ $DEPENDENCIES = low ] ; then composer update --prefer-dist --prefer-lowest --prefer-stable ; fi
     - if [ ! $DEPENDENCIES ] ; then composer update --prefer-dist ; fi
     - composer install