make enhancer services private

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no (but seems unrelated)
| Fixed tickets | https://github.com/symfony-cmf/SimpleCmsBundle/issues/66
| License       | MIT

requires https://github.com/sonata-project/SonataBlockBundle/pull/91 to have a tagged version

fix for regression from #169