Build phar using box

Fixes #532

``` bash
$ ./bin/build_phar
```

Trying to use the build phar in the phpspec directoy in which it was build will result in an redeclaration error, as it will try and load `vendor/autoload.php` and as the files in the phar and in the project are exactly the same, it will fail. So try it out in another project using phpspec :)