Reenable Notification Center

The command to disable the notification center

```
launchctl unload -w /System/Library/LaunchAgents/com.apple.notificationcenterui.plist 2> /dev/null
```

[doesn't work as expected](https://github.com/mathiasbynens/dotfiles/issues/476) on Yosemite and El Capitan. How do I reset the default behavior?

Also, I think it would be nice to add a comment in `.osx` that warns about this issue, as the comment `Disable Notification Center and remove the menu bar icon` is misleading for whoever is running a recent version of OS X.

Start by reviewing the Notification Center command and its surrounding comment in .osx, then reproduce the behavior on the affected macOS versions if available. Determine the documented reset behavior and update the comment or command so the default behavior is clear and verifiable.

Written by the indexing model from the issue text.