[4.0] Correct syntax in workbench composer.json stub

Version constraint should use asterisk (`*`) not `x` as that will only match dev versions (according to [Igor Wiedler](https://igor.io/) ([on IRC](http://colabti.org/irclogger/irclogger_log/composer?date=2013-10-23#l401))).

Additionally, the namespace matching string in `psr-0` should contain the final `\` to ensure the most correct match (see [composer docs](http://getcomposer.org/doc/04-schema.md#psr-0)).