RbConfig::LIMITS only provide after Ruby 2.5

Author: hsbt

.github/workflows/macos.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: macos-latest
     strategy:
       matrix:
-        ruby: [ '2.6.x', '2.5.x', '2.4.x' ]
+        ruby: [ '2.6.x', '2.5.x' ]
     steps:
     - uses: actions/checkout@master
     - name: Set up Ruby

.github/workflows/ubuntu.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: [ '2.6.x', '2.5.x', '2.4.x' ]
+        ruby: [ '2.6.x', '2.5.x' ]
     steps:
     - uses: actions/checkout@master
     - name: Set up Ruby

.github/workflows/windows.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: windows-latest
     strategy:
       matrix:
-        ruby: [ '2.6.x', '2.5.x', '2.4.x' ]
+        ruby: [ '2.6.x', '2.5.x' ]
     steps:
     - uses: actions/checkout@master
     - name: Set up Ruby

stringio.gemspec

@@ -23,7 +23,7 @@ Gem::Specification.new do |s|
   s.files = ["README.md", "ext/stringio/extconf.rb", "ext/stringio/stringio.c"]
   s.homepage = "https://github.com/ruby/stringio"
   s.licenses = ["BSD-2-Clause"]
-  s.required_ruby_version = ">= 2.4"
+  s.required_ruby_version = ">= 2.5"
   s.rubygems_version = "2.6.11"
   s.summary = "Pseudo IO on String"