GH-816: Simplify Validator configuration

[garyrussell]

Resolves #816

[artembilan]

Looks like we still need to add some optional or testCompile dependency:

/home/travis/build/spring-projects/spring-kafka/spring-kafka/src/test/java/org/springframework/kafka/annotation/EnableKafkaIntegrationTests.java:39: error: package javax.validation does not exist
import javax.validation.Valid;
                       ^
/home/travis/build/spring-projects/spring-kafka/spring-kafka/src/test/java/org/springframework/kafka/annotation/EnableKafkaIntegrationTests.java:40: error: package javax.validation does not exist
import javax.validation.ValidationException;
                       ^
/home/travis/build/spring-projects/spring-kafka/spring-kafka/src/test/java/org/springframework/kafka/annotation/EnableKafkaIntegrationTests.java:1539: error: cannot find symbol
		public void validatedListener(@Payload @Valid ValidatedClass val) {
		                                        ^
  symbol:   class Valid
  location: class Listener