Implement function for version shortcuts to use latest version

Add possibility to use version numbers like 5.3 instead of 5.3.17. Now phpbrew will pick the latest available version. Examples:

Installed versions:

* 5.5.4
* 5.4.10
* 5.3.5
* 5.3.4

Commands:

* `phpbrew switch 5.5` will switch to 5.5.4
* `phpbrew switch 5` will switch to 5.5.4
* `phpbrew use 5.3` will use 5.3.5

It is not available if you are using version numbers with the `php-` prefix.