DEV: Require ruby >= 2.5 (#46)

And update the CI workflow

.github/workflows/ci.yml

@@ -4,7 +4,6 @@ on:
   pull_request:
   push:
     branches:
-      - master
       - main
 
 jobs:
@@ -12,10 +11,12 @@ jobs:
     runs-on: ubuntu-latest
     name: "Ruby ${{ matrix.ruby }} / Failure allowed: ${{ matrix.experimental }}"
     continue-on-error: ${{ matrix.experimental }}
+    timeout-minutes: 5
+
     strategy:
       fail-fast: false
       matrix:
-        ruby: ["2.4", "2.5", "2.6", "2.7", "3.0"]
+        ruby: ["2.5", "2.6", "2.7", "3.0", "3.1"]
         experimental: [false]
         include:
           - ruby: "ruby-head"
@@ -24,9 +25,9 @@ jobs:
             experimental: true
           - ruby: "jruby-head"
             experimental: true
-          - ruby: "jruby-9.1.17.0"
+          - ruby: "jruby-9.2.20.1"
             experimental: true
-          - ruby: "jruby-9.2.13.0"
+          - ruby: "jruby-9.3.2.0"
             experimental: true
     steps:
       - uses: actions/checkout@v2

mini_mime.gemspec

@@ -21,6 +21,7 @@ Gem::Specification.new do |spec|
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
+  spec.required_ruby_version = ">= 2.5.0"
 
   spec.add_development_dependency "bundler"
   spec.add_development_dependency "rake"