Stick to 2.8 as the default target for the test suite.

Even though tere are no changes between running the test suite against
3.0 and 2.8, we will stick with the current stable Redis version as
the default target and switch to 3.0 as soon as it will be released.

[ci skip]

phpunit.xml.dist

@@ -27,7 +27,7 @@
 
     <php>
         <!-- Redis -->
-        <const name="REDIS_SERVER_VERSION" value="3.0" />
+        <const name="REDIS_SERVER_VERSION" value="2.8" />
         <const name="REDIS_SERVER_HOST" value="127.0.0.1" />
         <const name="REDIS_SERVER_PORT" value="6379" />
         <const name="REDIS_SERVER_DBNUM" value="15" />