Add ruby 3 support

Author: jcoyne

.github/workflows/super_diff.yml

@@ -9,6 +9,7 @@ jobs:
           - 2.5
           - 2.6
           - 2.7
+          - 3.0
         rails_appraisal:
           - rails_5_0
           - rails_5_1

super_diff.gemspec

@@ -12,7 +12,7 @@ Gem::Specification.new do |s|
     SuperDiff is a gem that hooks into RSpec to intelligently display the
     differences between two data structures of any type.
   DESC
-  s.required_ruby_version = "~> 2.4"
+  s.required_ruby_version = ">= 2.4"
 
   s.files         = ["README.md", "super_diff.gemspec"] + Dir["lib/**/*"]
   s.test_files    = Dir["spec/**/*"]