Update Boot plugin to latest snapshot

The latest Boot plugin has a number of useful features including the ability to create the executable dirt jar in place (instead of using the spring-xd-exec pseudo-project).

TODO: there's no reason spring-xd-dirt.jar (the non-executable version) needs to have application.yml in it, but the shell tests don't have an application.yml of their own so they will fail if we take it out of the DIRT jar now.