Make 2.6 the default server profile for the test suite too.

We leave 2.4 for Travis CI since their CI environment uses Redis 2.4.

[ci skip]

phpunit.xml.dist

@@ -27,7 +27,7 @@
 
     <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" />