use `installer :script` in 3 Casks

`installer` is a very late addition to the DSL because the form was changed from the original proposal (#6660, only available in the latest release 0.45.0).

However, `installer :script` is unlikely to be disruptive, as the change only touches 3 Casks, and the `method_missing` warning will be generated for anyone who tries to use these Casks without updating.

Part of the purpose here was to get rid of the old `caskroom_only`/`postflight` hack.  After this change, `caskroom_only` is used only once ([node-webkit.rb](https://github.com/caskroom/homebrew-cask/blob/c093d9df6abb1ebae4c999c56521142668665eba/Casks/node-webkit.rb#L9)).

Enforcing that `caskroom_only` may not be used in combination with other artifacts is still todo.