Drop to support Ruby 2.4

Author: hsbt

.github/workflows/ubuntu.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: [ '3.0', 2.7, 2.6, 2.5, 2.4, head, jruby-9.2.17.0, jruby-head ]
+        ruby: [ '3.0', 2.7, 2.6, 2.5, head, jruby-9.2.17.0, jruby-head ]
     steps:
     - uses: actions/checkout@v2
     - name: Set up Ruby

racc.gemspec

@@ -90,6 +90,7 @@ DESC
   ]
   s.require_paths = ["lib"]
   s.required_rubygems_version = Gem::Requirement.new(">= 0") if s.respond_to? :required_rubygems_version=
+  s.required_ruby_version = ">= 2.5"
   s.rdoc_options = ["--main", "README.rdoc"]
   s.extra_rdoc_files = [
     "README.ja.rdoc", "README.rdoc",