forward-compatibility to remove naming limitations on Casks

[rolandwalker]

Currently, Casks names are constrained by the need to form
valid Ruby class names. This change enables a new syntax,
in which the first line of a Cask will read like

cask :v1 => 'my-app' do

where :v1 indicates the version of the DSL spec.

The current PR merely converts such lines back to the old form

class MyApp < Cask

so the class name is still used behind the scenes, and the naming
limitation is still in effect in practice.

In upcoming months we will alter Casks/docs to use the new syntax,
and then change the backend code to remove the limitation.

Placing the DSL version in the header line means that #4870 would
not be needed.

[phinze]

Got your ping in IRC - This looks great!

I think in the future we may want to at least have the discussion about the redundancy between file name and the name mentioned in the argument, but for now this is a major step forward.

I am definitely +1 for getting a version into the DSL syntax. Makes future changes more straightforward.

I will try and take a look at the implementation in more detail tonight or tomorrow. But I'm definitely on board with the direction!

[rolandwalker]

This is the last item for #4688, so I'd like to merge (as soon as I get the encoding issue correct).