Update version to 2.84

Attempting ``brew cask install transmission`` produces the following error:

```
✓ → brew cask install transmission
==> Downloading https://transmission.cachefly.net/Transmission-2.83.dmg

curl: (22) The requested URL returned error: 404 Not Found
Error: Download failed on Cask 'transmission' with message: Download failed: https://transmission.cachefly.net/Transmission-2.83.dmg
```