shift Rubyies in CI: 1.9 - 2.2 -> 2.3 - 3.0

[swiknaba]

Shifts Ruby versions for CI from 1.9.3 - 2.2.2 to 2.3.8 - 3.0.0

2.3.8 is the oldest EOL version that you can still download on the official Ruby page.

Ruby 2.6 deprecated BigDecimal.new, hence specs for Ruby > 2.6 are failing with:

~/Sites/gems/symbolized/lib/symbolized/core_ext/object/duplicable.rb:86:in `<class:BigDecimal>': undefined method `new' for BigDecimal:Class (NoMethodError)
	from ~/Sites/gems/symbolized/lib/symbolized/core_ext/object/duplicable.rb:81:in `<top (required)>'
	from ~/Sites/gems/symbolized/lib/symbolized/core_ext/object/deep_dup.rb:2:in `require'
	from ~/Sites/gems/symbolized/lib/symbolized/core_ext/object/deep_dup.rb:2:in `<top (required)>'
	from ~/Sites/gems/symbolized/test/core_ext/hash_ext_test.rb:2:in `require'
	from ~/Sites/gems/symbolized/test/core_ext/hash_ext_test.rb:2:in `<top (required)>'
	from ~/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/rake-13.0.3/lib/rake/rake_test_loader.rb:17:in `require'
	from ~/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/rake-13.0.3/lib/rake/rake_test_loader.rb:17:in `block in <main>'
	from ~/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/rake-13.0.3/lib/rake/rake_test_loader.rb:5:in `select'
	from ~/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/rake-13.0.3/lib/rake/rake_test_loader.rb:5:in `<main>'

I've fixed that in a separate PR: #5

locally, specs are passing up until Ruby 2.6.6 ✅

53 runs, 219 assertions, 0 failures, 0 errors, 0 skips