register doctrine alias mapper pass

 to support CmfContentBundle:StaticContent shortcut

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | travis
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This is defensive, so it does not fail if the compiler pass is not yet there. but only makes sense if https://github.com/doctrine/DoctrinePHPCRBundle/pull/146 is merged (actually if we merge that as-is, we need to check for the base class of the pass as well, as the symfony version could be too old)