update composer.json, integrating proposal for seoBundle

first change what i have done is a update for the composer.json. Put all sonata dependencies to clear versions, avoiding the ~2.2@dev Version of the sonata-core-bundle.
I think i found a balanced setting. It works for now.

The second change is my proposal for the SeoBundle. Integrated it into the composer.json, created an example in the fixture loaders and added the setting. 
| Bug fix?      | yes the first part
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | https://github.com/symfony-cmf/cmf-sandbox/issues/232, https://github.com/symfony-cmf/ContentBundle/issues/99
| License       | MIT
| Doc PR        | not set yet