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.
Add the Spring IO Platform plugin
INT-3396: Polishing

JIRA: https://jira.spring.io/browse/INT-3396
Updates for latest snapshots of the Spring IO plugin

Update the configuration to use the renamed plugin. Add a dependency in
the springIoVersions configuration that points to the versions published
by Spring IO.