Restore commons-logging dependency for spring-boot

Restore the dependency on commons-logging (transitively via spring-core)
for spring-boot. This means that we are not tied directly to SLF4J, but
it is still an option that can be used via `jcl-over-slf4j`.

The `spring-boot-starter-parent` continues to replace `commons-logging`
with `jcl-over-slf4j`.

Fixes gh-981

spring-boot-tools/spring-boot-loader-tools/.gitignore

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+/target