Drop thread_safe and concurrent-ruby dependencies

[slonopotamus]

Seems like they serve no purpose in asciidoctor-reveal.js and most likely are artifacts from ancient Ruby versions

[ggrossetie]

Seems like they serve no purpose in asciidoctor-reveal.js and most likely are artifacts from ancient Ruby versions

I think you are right!
For reference, Dan said:

For compatibility with 1.5.8, you should add a dependency on the concurrent-ruby gem. You can still depend on thread_safe too. As long as both gems are present, it will work on both 1.5.7 and 1.5.8 without any warnings.

Asciidoctor core has a policy of having 0 dependencies. On top of that, it's only a recommended dependency, not a required one.

(^^^ speaking about thread_safe dependency)

And, btw, it's now concurrent-ruby, not thread_safe. Though, for it to work without warning on both 1.5.7 and 1.5.8, you'll need both on the load path.

[ggrossetie]

Thanks, less dependencies is 💚