Fix PHP version in travis build

.travis.yml

@@ -13,7 +13,7 @@ matrix:
   include:
     - php: 5.6
       env: DRIVER_VERSION=stable LEGACY_DRIVER_VERSION=stable
-    - php: 5.5
+    - php: 5.6
       env: DRIVER_VERSION=1.1.0 COMPOSER_FLAGS="--prefer-dist --prefer-lowest"
 
 addons: