Updated path to Kafka

Tests are failing because the script cannot fetch an Apache Kafka tarball. Updated path to the correct one

.travis.yml

@@ -23,7 +23,7 @@ before_install:
   - mkdir -p data/db
   - mongod --dbpath=data/db > /dev/null &
   - sleep 5
-  - wget http://www.us.apache.org/dist/kafka/0.9.0.1/kafka_2.11-0.9.0.1.tgz -O kafka.tgz
+  - wget http://www.us.apache.org/dist/kafka/0.11.0.2/kafka_2.12-0.11.0.2.tgz -O kafka.tgz
   - mkdir -p kafka && tar xzf kafka.tgz -C kafka --strip-components 1
   - nohup bash -c "cd kafka && bin/zookeeper-server-start.sh config/zookeeper.properties &"
   - sleep 10