Clarify supported ruby versions in gemspec

Author: kachick

erb.gemspec

@@ -14,7 +14,7 @@ Gem::Specification.new do |spec|
   spec.summary       = %q{An easy to use but powerful templating system for Ruby.}
   spec.description   = %q{An easy to use but powerful templating system for Ruby.}
   spec.homepage      = 'https://github.com/ruby/erb'
-  spec.required_ruby_version = Gem::Requirement.new('>= 2.3.0')
+  spec.required_ruby_version = Gem::Requirement.new('>= 2.5.0')
   spec.licenses      = ['Ruby', 'BSD-2-Clause']
 
   spec.metadata['homepage_uri'] = spec.homepage