Updated gemspec to require Ruby 2.3 or higher

ruby/google-protobuf.gemspec

@@ -20,6 +20,7 @@ Gem::Specification.new do |s|
   s.test_files  = ["tests/basic.rb",
                   "tests/stress.rb",
                   "tests/generated_code_test.rb"]
+  s.required_ruby_version = '>= 2.3'
   s.add_development_dependency "rake-compiler", "~> 0.9.5"
   s.add_development_dependency "test-unit", '~> 3.0', '>= 3.0.9'
   s.add_development_dependency "rubygems-tasks", "~> 0.2.4"