Add Ruby 3.x to CI testing

Author: tatethurston

.travis.yml

@@ -8,6 +8,10 @@ rvm:
   - 2.5
   - 2.6
   - 2.7
+  - 3.0
+  - 3.1
+  - 3.2
+  - 3.3
   - jruby-9.0.5.0
 jdk:
   - openjdk8

cassandra-driver.gemspec

@@ -34,4 +34,5 @@ Gem::Specification.new do |s|
 
   s.add_development_dependency 'bundler', '~> 1.6'
   s.add_development_dependency 'rake', '~> 13.0'
+  s.add_development_dependency 'sorted_set', '~> 1.0'
 end