Updated the installation instructions to Symfony 2.5+
feature #3874 Updated the installation instructions for Symfony 2.5+ (javiereguiluz)

This PR was merged into the 2.5 branch.

Discussion
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Updated the installation instructions for Symfony 2.5+

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | 2.5+
| Fixed tickets | #3748

Commits
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83bb723 Updated the installation instructions to Symfony 2.5+