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.