[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.
Hi @stof,
Ok, its my first issue, so you are probably right, the thing is, if you setup your vendor folder in other folder outside the defaults one, symfonyrequirements stop. 
So , I tried to see if there is another vendor dir, maybe this could fix it?
return $composerJson->config->{'vendor-dir'}."/composer" , so we assume the deps has been installed?
BTW: I did this in order to gain speed on vagrant instances: http://by-examples.net/2014/12/09/symfony2-on-vagrant.html  Thanks¡