blockbundle dropped the content_basepath configuration

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | adjust to https://github.com/symfony-cmf/BlockBundle/pull/131
| License       | MIT

This needs to be merged right before tagging the next release.