Fix wrong driver version in travis config

.travis.yml

@@ -14,7 +14,7 @@ matrix:
     - php: 5.6
       env: DRIVER_VERSION=stable LEGACY_DRIVER_VERSION=stable
     - php: 5.6
-      env: DRIVER_VERSION=1.1.0 COMPOSER_FLAGS="--prefer-dist --prefer-lowest"
+      env: DRIVER_VERSION=stable COMPOSER_FLAGS="--prefer-dist --prefer-lowest"
 
 addons:
   apt: