Added support for SensioGeneratorBundle 2.5 in GenerateAdminCommand

Kept BC compatibility with SensioGeneratorBundle 2.3 for Symfony 2.3

| Q             | A
| ------------- | ---
| Bug fix?      | yes (fixing BC break in SensioGeneratorBundle)
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #2604, #2626
| License       | MIT