Drop ruby 1.9.3 support

[zetachang]

Given that

• Opal is already depending on ruby version >= 2.0 on master (opal/opal@5990ac3)
• Support for ruby 1.9.3 has already ended about 1 year. https://www.ruby-lang.org/en/news/2015/02/23/support-for-ruby-1-9-3-has-ended/

So in the next release of hyper-react, it will require ruby >= 2.0.

[catmando]

Agreed.

[janbiedermann]

1.0.0-lap18 is tested and targeted with and for ruby 2.4.3 and ruby 2.5.0