Use "stable" instead of version for pecl installs

.travis.yml

@@ -7,12 +7,12 @@ php:
   - 7.0
 
 env:
-  - DRIVER_VERSION=1.1.1
+  - DRIVER_VERSION=stable
 
 matrix:
   include:
     - php: 5.6
-      env: DRIVER_VERSION=1.1.1 LEGACY_DRIVER_VERSION=1.6.12
+      env: DRIVER_VERSION=stable LEGACY_DRIVER_VERSION=stable
 
 addons:
   apt: