Update .travis.yml

.travis.yml

@@ -13,7 +13,7 @@ env:
   - SYMFONY_VERSION=dev-master
 
 before_script:
-  - composer require symfony/symfony:${SYMFONY_VERSION} --prefer-source
+  - composer require symfony/symfony:${SYMFONY_VERSION} --no-update
   - composer install --dev --prefer-source
 
 script: phpunit
@@ -23,4 +23,4 @@ notifications:
 
 matrix:
   allow_failures:
-    - php: hhvm
+    - php: hhvm