Add version.rb generation on rake update

[Morozzzko]

Hello there!

I've been fooling around with different versions of intro.js-rails on my local server and found it extremely uncomfortable to update the version.rb manually.

So I decided to generate it.

This PR proposes the automatic version.rb generation based on the selected intro.js tag.

[Morozzzko]

Please note that this PR requires Ruby 2.3 (since it uses squiggly heredoc syntax).

It should not be a problem unless you really need to run rake update on older rubies