Update highline requirement from ~> 2.0 to >= 2, < 4 (#132)

Updates the requirements on [highline](https://github.com/JEG2/highline) to permit the latest version.
- [Changelog](https://github.com/JEG2/highline/blob/master/Changelog.md)
- [Commits](JEG2/highline@v2.0.0...v3.0.0)

---
updated-dependencies:
- dependency-name: highline
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

cf-uaac.gemspec

@@ -38,7 +38,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'simplecov-rcov', '~> 0.3.1'
   s.add_development_dependency 'ci_reporter', '~> 2.0'
   s.add_development_dependency 'ci_reporter_rspec', '~> 1.0'
-  s.add_runtime_dependency 'highline', '~> 2.0'
+  s.add_runtime_dependency 'highline', '>= 2', '< 4'
   s.add_runtime_dependency 'eventmachine', '~> 1.2'
   s.add_runtime_dependency 'launchy', '~> 2.5'
   s.add_runtime_dependency 'em-http-request', '~> 1.1', '>= 1.1.2'