Add new directory structure hook
feature #641 Add new directory structure hook (romainneutron)

This PR was squashed before being merged into the 2.4-dev branch (closes #641).

Discussion
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Add new directory structure hook

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Fixed tickets | #584
| License       | MIT

This requires https://github.com/sensiolabs/SensioDistributionBundle/pull/118 to be merged

Commits
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bffde32 Add new directory structure hook
fixed version in composer.json
updated version to 2.6
Merge remote-tracking branch 'symfony/master'
Fixed optional error handling in production