Added support for Sensio Generator 2.2 (to not break support for Symfony 2.2 projects 

Symfony 2.2 projects has dependency on Sensio Generator 2.2. Current master of Sonata should support Symfony 2.2 and 2.3 projects, because we have no branch for 2.2. This will fix the problem with different incompatible versions of SensioGenerator.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT