change version numbers in installation notes to be in line with the documented Symfony version
minor #3882 change version numbers in installation notes to be in line with the docu... (xabbuh)

This PR was submitted for the master branch but it was merged into the 2.5 branch instead (closes #3882).

Discussion
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change version numbers in installation notes to be in line with the docu...

...mented Symfony version

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.5
| Fixed tickets |

This PR updates the version constraints in the installation instructions as described [here](http://symfony.com/doc/current/contributing/documentation/overview.html#when-a-new-branch-is-created-for-a-release):

> Change all version and master references to the correct version (e.g. 2.3). For example, in installation chapters, we reference the version you should use for installation. As an example, see the changes made in PR #2688.

Commits
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f77392a change version numbers in installation notes to be in line with the documented Symfony version
Merge remote-tracking branch 'origin/2.5' into 2.5