Updated version number and sha1 digest in formula for phpDocumentor version 2.0.0b7

The phpDocumentor formula downloads a phar file from http://www.phpdoc.org/phpDocumentor.phar and the downloaded version is the latest version available (currently 2.0.0b7). phpDocumentor formula specifies 2.0.0b6, the sha1 digest mismatches thus it can't be installed. This change updates the sha1 digest to match the latest version.