[DistributionBundle] Fix real vendor dir , reading from composer.json file

| Q             | A
| ------------- | ---
| Bug fix?      | [yes]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [-]
| Fixed tickets | #184
| License       | MIT
| Doc PR        | []

Couldnt find the test, I tested manually replacing the SymfonyRequirements in a 2.6 distribution.
It's my first PR, so, i hope this can help. Thanks.
Can you make this method private?