Update thor requirement from ~> 0.18 to >= 0.18, < 2.0

Updates the requirements on [thor](https://github.com/wycats/thor) to permit the latest version.
- [Release notes](https://github.com/wycats/thor/releases)
- [Changelog](https://github.com/erikhuda/thor/blob/master/CHANGELOG.md)
- [Commits](rails/thor@v0.18.0...v0.20.3)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

omnibus.gemspec

@@ -28,7 +28,7 @@ Gem::Specification.new do |gem|
   gem.add_dependency "mixlib-shellout",  ">= 2.0", "< 4.0"
   gem.add_dependency "ohai",             ">= 13", "< 16"
   gem.add_dependency "ruby-progressbar", "~> 1.7"
-  gem.add_dependency "thor",             "~> 0.18"
+  gem.add_dependency "thor",             ">= 0.18", "< 2.0"
   gem.add_dependency "license_scout",    "~> 1.0"
 
   gem.add_dependency "mixlib-versioning"