Set the test suite to run against Redis 2.8 on Travis CI.

Please note that the distributed phpunit.xml.dist still targets Redis
2.6 to match the default server profile used by Predis v0.8.

phpunit.xml.travisci

@@ -31,7 +31,7 @@
 
     <php>
         <!-- Redis -->
-        <const name="REDIS_SERVER_VERSION" value="2.6" />
+        <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" />