Added Vagrant configuration cookbook

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | n/a

A new cookbook article was added with an example Vagrant configuration that can be used with the Symfony Standard Edition.
It was a good enough reason for me to keep with the Vagrant documentation at the time.  That way the developer could correlate the Vagrant documentation examples with the lines in this configuration file.  Also, the `precise32` and `precise64` base boxes were prepared by the same company (Hashicorp) that developed Vagrant, so it would make sense that they would try and make sure that they work well for others to use.  While that version of the operating system might seem a bit "old" now, it could also be considered "stable" to some.

I'm not sure if it will be supported by Symfony 3.  I think I saw some other Symfony repository Pull Request discussions and even a poll to determine which minimum PHP version to use (PHP 5.5?).  In any case, Symfony 3 is still a ways away from the time when most developers will have a stable version in their hands, so it might be better to focus on Symfony 2 for now with this cookbook.