Simplifying phar builds

Hi,

Current phar build goes like this:

> You need ant to build the phar. Just checkout a tagged release and run ant phar where you would normally run composer install. The phar should end up in the build directory.

I know you guys are using **ant** to do much more than build a phar file (at least the xml config looks like to be doing a lot), but sometimes a poor user just wants to apply a temporary patch and build a new phpunit binary without :coffee:

So, here is a simplified pure PHP build approach using [php-box](https://github.com/kherge/php-box). Inside `phpunit` project dir:

0. composer install
0. box build --verbose

After following these two steps a `phpunit.phar` will be available at `/build` folder.

Also notice the box build will produce a ~2,5 MB phar file instead of a ~3,2 MB, like we get with current :ant: build. I guess a lot of superfluous files are being put into current phar releases and it could be improved too.

Cheers!