Add cask for Avast Mac Security

Create a cask for Avast Mac Security. Not entirely sure if the
uninstall stanza is correct. The dmg comes with an uninstaller
with the filename 'com.avast.uninstall.app', but I can't find
anything in the docs about using a pre-packaged uninstaller.

Remove broken uninstall stanza

Add uninstall stanza suggested by @rolandwalker

This stanza seems to work. Application and support files all removed
after uninstall.
Merge pull request #7675 from jawshooah/add-avast-cask

Add cask for Avast Mac Security