Put 1.9.3 support back

[pitr-ch]

fix #768

[nijikon]

@pitr-ch as I appreciate you fixing this, I don't think 1.9.3 should still be supported. Support for it ended at 23rd of February 2015, https://www.ruby-lang.org/en/news/2015/02/23/support-for-ruby-1-9-3-has-ended/. People should upgrade or just use the last available version of a gem for their ruby version. We should not support old stuff for forever.

[nijikon]

To be honest, we should support version >= 2.3, people should not be encouraged to run old ruby versions.

[pitr-ch]

With 2 indirections, 20% of all existing gems depend on concurrent-ruby. I have to be extra careful regarding compatibility. I plan to drop the support for old Ruby version eventually but I want to communicate it first with authors of the libraries depending on concurrent-ruby. Not dropping it all of a sudden as I did in 1.1.

[nijikon]

Agreed, I would opt for bumping to 2.0 and drop support for < 2.3.

[nijikon]

Or maybe 2.0 is too big of a bump, I guess 1.1 would be fine for me if we would communicate it in the changelog as breaking changes.