XD-2409 Changing correlation-strategy-expression

- this will avoid some issues ecountered with SpEL (SPR-12502)

- using "payload.class.name" seems to work in all cases
XD-2387 Kafka AcceptanceTests
XD-2333 Add Kafka integration test

- Add kafka test support class that checks for kafka server availablity
  on localhost
- This uses zk server available on default connect string `localhost:2181`
- Add test for verifying kafka source and sink having both the source/sink
  on the same test case will make sure the topic that gets the data by
  kafka sink is actually read by kafka source
- Handle Zk connect exceptions
- Fix create/delete topic in Before/After
- Remove explicit sleep by setting kafka source consumer
  timeout to -1 (blocking)
- Fix import statements
XD-2426: Use new Travis CI build env, add caching

- test cache behavior
- Add debug
- use -PwikiBranch=origin/master
XD-2386 Acceptance tests for tcp-client source

connect to a nc (netcat) server that returns the following value UOHSO638abna9beqw12.
The netcat can be run using the following:
(Mac)
while true; do nc -l -c  8782 < index.html; done
(*nix)
while true; do nc -l -C  8782 < index.html; done

where index contains UOHSO638abna9beqw12

See spring-xd-integration-test/src/scripts for scripts that
launch the nc process as described above.
XD-2475 MessageBus Producer Batching

JIRA: https://jira.spring.io/browse/XD-2475

Rabbit: Use a `BatchingRabbitTemplate` if batching is enabled.
Kafka: Set properties

Tested with:

    xd:>stream create foo --definition "time | log"
    xd:>stream deploy --name foo --properties "module.time.producer.batchingEnabled=true,module.time.producer.batchSize=6"

Observe messages appear in log in batches of 6.

    xd:>stream undeploy foo

Observe partial batch is flushed.

(Also added RabbitMessageBus test).