perf: fix warmup by doing request forwarding - exercises the full stack!

Other fixes:
- this also fixes a build issues with the mapreduce library which now depends on guava 15 by removing the exclusions workaround added in pull #237 

```
[INFO] java.lang.NoSuchMethodError: com.google.common.base.Stopwatch.createUnstarted()Lcom/google/common/base/Stopwatch;
[INFO] 	at com.google.appengine.tools.cloudstorage.RetryHelper.runWithRetries(RetryHelper.java:123)
[INFO] 	at com.google.appengine.tools.mapreduce.MapReduceJob.checkQueueSettings(MapReduceJob.java:110)
[INFO] 	at com.google.appengine.tools.mapreduce.MapReduceJob.start(MapReduceJob.java:100)
[INFO] 	at org.karmaexchange.task.ComputeLeaderboardServlet.startComputeLeaderboardMapReduce(ComputeLeaderboardServlet.java:35)
```