Specify minimum Ruby version.

Remove jruby-9.1.17.0 from CI

Author: marcandre

.github/workflows/ubuntu-rvm.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: [ 'jruby-9.2.6.0', 'jruby-9.1.17.0', 'ruby-head' ]
+        ruby: [ 'jruby-9.2.6.0', 'ruby-head' ]
     steps:
     - uses: actions/checkout@master
     - name: Set up RVM

forwardable.gemspec

@@ -18,6 +18,7 @@ Gem::Specification.new do |spec|
   spec.homepage      = "https://github.com/ruby/forwardable"
   spec.license       = "BSD-2-Clause"
 
+  spec.required_ruby_version = '>= 2.4.0'
   spec.files         = ["forwardable.gemspec", "lib/forwardable.rb", "lib/forwardable/impl.rb", "lib/forwardable/version.rb"]
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }