[Core] fix symfony-service-definiton-validator version

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #2143
| Refs tickets  | 
| License       | MIT
| Doc PR        |

Update the `matthiasnoback/symfony-service-definition-validator`
dependency to a version that supports the new factory syntax introduced
in Symfony 2.6.