Update all relevant starter POMs to include a `spring-core` dependency
with an exclusion on `commons-logging`. This prevents `commons-logging`
and `jcl-over-slf4j` from both being on the classpath.
Also add enforcer rules to ensure that commons-logging doesn't sneak
back in, and that there is no dependency convergence. (some additional
libraries were required in spring-boot-dependencies)
Tested with a sample maven project as well as using the `spring jar`
command.
Fixes gh-985
Phillip Webb
Andy Wilkinson
Dave Syer