[DistributionBundle] Fix real vendor dir , reading from composer.json file
bug #185 [DistributionBundle] Fix real vendor dir , reading from composer.json file (karlosagudo)

This PR was squashed before being merged into the 3.0.x-dev branch (closes #185).

Discussion
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[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.

Commits
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798ddcd [DistributionBundle] Fix real vendor dir , reading from composer.json file