Use spring Boot events to get exit code

Spring Boot now provides the result of an ExitCodeExceptionMapper via an
event called ExitCodeEvent.  This commit takes advantage of this new
event.

Resolves spring-cloud/spring-cloud-task#48

spring-cloud-task-core/src/test/java/org/springframework/cloud/task/listener/TaskLifecycleListenerTests.java

@@ -99,6 +99,7 @@ public class TaskLifecycleListenerTests {
 		context.refresh();
 		RuntimeException exception = new RuntimeException("This was expected");
 		context.publishEvent(new ApplicationFailedEvent(new SpringApplication(), new String[0], context, exception));
+		context.publishEvent(new ContextClosedEvent(context));
 
 		verifyTaskExecution(0, true, 1, exception);
 	}