Fix env variable in travis.yaml

SYMFONY_DEPS_VERSION=3 was never evaluated correctly as the before_scripts are comparing with 3.0
The test are installing dev-master for some dependencies which might not be 3.0

.travis.yml

@@ -29,6 +29,6 @@ matrix:
         - php: 5.6
           env: SYMFONY_DEPS_VERSION=2.8
         - php: 5.6
-          env: SYMFONY_DEPS_VERSION=3
+          env: SYMFONY_DEPS_VERSION=3.0
         - php: 7.0
         - php: hhvm