update supported versions

* ruby >= 2.4
* address ruby 2.7 deprecation warnings

lib/rex/text/unicode.rb

@@ -220,7 +220,7 @@ module Rex
     # @return [String] a UTF-8 equivalent
     # @note This method will discard invalid characters
     def self.to_utf8(str)
-      str.encode('utf-8', { :invalid => :replace, :undef => :replace, :replace => '' })
+      str.encode('utf-8', **{ :invalid => :replace, :undef => :replace, :replace => '' })
     end
 
     #

rex-text.gemspec

@@ -18,7 +18,7 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
-  spec.required_ruby_version = '>= 2.2.0'
+  spec.required_ruby_version = '>= 2.4.0'
 
   spec.add_development_dependency "rake"
   spec.add_development_dependency "rspec"