INT-3397 Add spring-integration-bom

JIRA: https://jira.spring.io/browse/INT-3397

Similar to Spring Framework's; see
https://spring.io/blog/2013/12/03/spring-framework-4-0-rc2-available
Update preface docs for Java 8 compatibility

Reference docs in preface mention Spring Integration compatibility with
Java 8, once it's released. Since Java 8 is released, the extra note is
no longer relevant and should be removed.

This patch removes no longer relevant note of pending Java 8 release.

JIRA: https://jira.spring.io/browse/INT-3400
INT-3400 Fix Preface for 4.0.x

JIRA: https://jira.spring.io/browse/INT-3400

Preface showed 3.0.x compatibility instead of 4.0.x.
INT-3394: Use Jedis Client in Tests

JIRA: https://jira.spring.io/browse/INT-3394

INT-3394: Restore `awaitContainerSubscribed`
INT-3398 Polish Javadocs

JIRA: https://jira.spring.io/browse/INT-3398
INT-3399 Update spring-retry to 1.1.0

JIRA: https://jira.spring.io/browse/INT-3399