Update slack symlink

Similar to #4487

Seems that when Slack is unarchived it is named "Slack 0.44.2.app" instead of the "Slack.app" that is currently expected.

```
$ /usr/local/bin/brew cask install slack
==> Downloading http://slack.com/ssb/download-osx
Already downloaded: /Library/Caches/Homebrew/slack-latest
Error: It seems the symlink source is not there: '/opt/homebrew-cask/Caskroom/slack/latest/Slack.app'
```

```
$ /usr/local/bin/brew cask install test/slack.rb 
==> Downloading http://slack.com/ssb/download-osx
Already downloaded: /Library/Caches/Homebrew/slack-latest
==> Symlinking App 'Slack 0.44.2.app' to '/Users/jmaclean/Applications/Slack 0.44.2.app'
slack installed to '/opt/homebrew-cask/Caskroom/slack/latest' (181 files, 8.4M)
```