Alias master branch as 2.8.x for composer

The `master` branch is currently only available by requiring `dev-master` in a downstream composer.json file. This is not recommended because:
* It creates a hard dependency on your branch naming strategy
* It is too loose, and may expose end-users to unexpected major version upgrades
* It can cause dependency resolution conflicts if other packages impose a numeric version constraint on phing/phing

By adding the branch alias, users can either require `"phing/phing": "2.8.*@dev"` directly, or can set an overall `minimum-stability` flag in their composer.json to accept the dev version of the package. Anyone already using `dev-master` can continue to do so.

Note that you would then need to update the branch-alias once master becomes the development branch for eg  2.9.x, 3.0.x etc.