Fix MacPass cask: update .app path for v0.4.1-alpha

In the previous version of MacPass ([0.3.8-alpha](https://github.com/mstarke/MacPass/releases/download/0.3.8-alpha/MacPass-0.3.8-alpha.zip)) the .app was inside a directory within the zip. In [0.4.1-alpha](https://github.com/mstarke/MacPass/releases/download/0.4.1-alpha/MacPass-0.4.1-alpha.zip), it is not, so this cask fails to install.