minor #4568 Update Symfony reference to 2.6 version (dangarzon)

This PR was merged into the 2.6 branch.

Discussion
----------

Update Symfony reference to 2.6 version

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.6 only
| Fixed tickets |

It's a bit unpractical having to update the version by hand each time a mayor version is released... and hard to remember.

Can this be resolved with a parameter (or something similar) common to the whole documentation??

Commits
-------

27cb445 Update Symfony reference to 2.6 version

book/installation.rst

@@ -59,7 +59,7 @@ Distribution:
 
 .. code-block:: bash
 
-    $ php composer.phar create-project symfony/framework-standard-edition /path/to/webroot/Symfony '~2.5'
+    $ php composer.phar create-project symfony/framework-standard-edition /path/to/webroot/Symfony '~2.6'
 
 .. tip::
 
@@ -112,10 +112,10 @@ one of the following commands (replacing ``###`` with your actual filename):
 .. code-block:: bash
 
     # for .tgz file
-    $ tar zxvf Symfony_Standard_Vendors_2.5.###.tgz
+    $ tar zxvf Symfony_Standard_Vendors_2.6.###.tgz
 
     # for a .zip file
-    $ unzip Symfony_Standard_Vendors_2.5.###.zip
+    $ unzip Symfony_Standard_Vendors_2.6.###.zip
 
 If you've downloaded "without vendors", you'll definitely need to read the
 next section.