DSL: change container_type to extensible container

Late addition to DSL 1.0 (oversight in the roadmap)

References: #4688 

New syntax:
```ruby
container :type => :naked
```

This is obviously more extensible than `container_type`, but I simply forgot to make
this amendment in #4688.

An example of what else might possibly be added in this stanza is renaming:
```ruby
container :rename => 'bankid-latest.pkg'
```
So that we wouldn't need this [hack](https://github.com/caskroom/homebrew-cask/pull/6002#issuecomment-54649885).  No sure yet; just an example.

@caskroom/maintainers, as this only affects 5 Casks in the main repo, I am comfortable
pushing in this change at the last minute and including it in DSL 1.0.  The chance that
a user will actually be inconvenienced is very small.