Increase kafka consumers cache size

Fixes #21

src/main/scala/com/mozilla/telemetry/streaming/ErrorAggregator.scala

@@ -197,6 +197,7 @@ object ErrorAggregator {
       .format("kafka")
       .option("kafka.bootstrap.servers", opts.kafkaBroker())
       .option("kafka.max.partition.fetch.bytes", 8 * 1024 * 1024) // 8MB
+      .option("spark.streaming.kafka.consumer.cache.maxCapacity", 1000)
       .option("subscribe", "telemetry")
       .option("startingOffsets", "latest")
       .load()