Removed support for SensioGeneratorBundle 2.2

SonataAdmin (master branch) is now supporting only Symfony >=2.3, so we can safely drop support for 
SensioGeneratorBundle 2.2 (needed for Symfony 2.2 support).

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no for SonataAdmin; yes for support SensioGeneratorBundle 2.2
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT