Bump Redis version used in tests on Travis CI.

It seems like Travis CI switched silently to Redis 2.6, which means
that we can enable the full test suite of Predis.

phpunit.xml.travisci

@@ -31,7 +31,7 @@
 
     <php>
         <!-- Redis -->
-        <const name="REDIS_SERVER_VERSION" value="2.4" />
+        <const name="REDIS_SERVER_VERSION" value="2.6" />
         <const name="REDIS_SERVER_HOST" value="127.0.0.1" />
         <const name="REDIS_SERVER_PORT" value="6379" />
         <const name="REDIS_SERVER_DBNUM" value="15" />