Allow highline 3.x

highline 3.0 dropped support for old Ruby versions and reworked its
internal API, but the public surface kafo relies on (HighLine.new,
agree/ask/choose/say, HighLine::String, terminal/terminal_size) is
unchanged.

Author: ogajduse

kafo.gemspec

@@ -34,7 +34,7 @@ Gem::Specification.new do |spec|
   # CLI interface
   spec.add_dependency 'clamp', '>= 1.3.1', '< 2'
   # interactive mode
-  spec.add_dependency 'highline', '>= 1.6.21', '< 3.0'
+  spec.add_dependency 'highline', '>= 1.6.21', '< 4.0'
   # ruby progress bar
   spec.add_dependency 'powerbar'
 end