Update unicode-display_width dependency

Support using v3 of unicode-display_width, inline with rubocop, so that we're not forcing users to downgrade

Author: KieranP

tty-progressbar.gemspec

@@ -31,7 +31,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency "strings-ansi", "~> 0.2"
   spec.add_dependency "tty-cursor", "~> 0.7"
   spec.add_dependency "tty-screen", "~> 0.8"
-  spec.add_dependency "unicode-display_width", ">= 1.6", "< 3.0"
+  spec.add_dependency "unicode-display_width", ">= 2.4.0", "< 4.0"
 
   spec.add_development_dependency "rake"
   spec.add_development_dependency "rspec", ">= 3.0"