Improved the way we display the executed isntaller command

This PR tries to solve #80 for the most used case.  Around 80% of users use Linux or Mac and most of them will install the installer in `/usr/local/bin/`, so we can easily improve commands' output.
Using `str_replace()` here is dangerous if the user stores the installer, for example, in `/symfony/symfony`. `executedCommand` will then be an empty string in the end. You could use `basename()` instead.