fix env var name

Author: xabbuh

.travis.yml

@@ -31,7 +31,7 @@ matrix:
 before_install:
   - if [ "$TRAVIS_PHP_VERSION" != "hhvm" ]; then phpenv config-rm xdebug.ini; fi
   - if [ "$DEPENDENCIES" != "" ]; then composer config minimum-stability $DEPENDENCIES; fi
-  - if [ "$SYMFONY_VERSION" != "" ]; then composer require --dev --no-update symfony/lts=$SYMFONY_VERSION; fi
+  - if [ "$SYMFONY_LTS" != "" ]; then composer require --dev --no-update symfony/lts=$SYMFONY_LTS; fi
 
 install: composer update $COMPOSER_FLAGS --prefer-dist