Update installation.rst

Installing the bundle I got 
  The service "sonata.classification.api.form.type.category" has a dependency on a non-existent service "jms_serializer.metadata_factory".

The library was present but not loaded on AppKernel.
Adding it solved the problem...
Merge pull request #202 from mmf-amarcos/patch-1

Missing jmsserializer instance on appkerner. Update installation.rst