Drop support for ruby < 3.1

Author: k-tsj

.github/workflows/ci.yml

@@ -5,7 +5,7 @@ jobs:
     uses: ruby/actions/.github/workflows/ruby_versions.yml@master
     with:
       engine: cruby
-      min_version: 2.5
+      min_version: 3.1
   test:
     needs: ruby-versions
     name: >-
@@ -21,9 +21,6 @@ jobs:
             os: "ubuntu-latest"
             TEST_SYMLINK: yes
             rubyopt: "--enable-frozen-string-literal"
-        exclude:
-          - ruby-version: "2.5"
-            os: "macos-latest"
     runs-on: ${{ matrix.os }}
     env:
       TEST_SYMLINK: ${{ matrix.TEST_SYMLINK }}

Gemfile

@@ -12,7 +12,6 @@ group :development do
   gem 'test-unit'
   gem 'rake'
   gem 'simplecov'
-  gem 'bundler'
   gem 'irb', '>= 1.3.1'
   gem 'benchmark-ips'
 end

README.md

@@ -24,11 +24,11 @@ Use following test frameworks or extensions instead.
 * [power_p](https://github.com/k-tsj/power_p)
 
 ## Requirement
-* CRuby 2.5+
+* CRuby 3.1+
 
 ## Configuration
 To colorize output messages, add <code>require "power_assert/colorize"</code> to your code.
-(It requires CRuby 3.0.1+ or irb 1.3.1+)
+(It requires irb 1.3.1+)
 
 ## Known Limitations
 * Expressions must be put in one line. Expressions with folded long lines produce nothing report, e.g.: