Update ferrum requirement from ~> 0.13.0 to >= 0.13, < 0.16

Updates the requirements on [ferrum](https://github.com/rubycdp/ferrum) to permit the latest version.
- [Release notes](https://github.com/rubycdp/ferrum/releases)
- [Changelog](https://github.com/rubycdp/ferrum/blob/main/CHANGELOG.md)
- [Commits](rubycdp/ferrum@v0.13...v0.15)

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

Signed-off-by: dependabot[bot] <[email protected]>

cuprite.gemspec

@@ -25,5 +25,5 @@ Gem::Specification.new do |s|
   s.required_ruby_version = ">= 2.7.0"
 
   s.add_runtime_dependency "capybara", "~> 3.0"
-  s.add_runtime_dependency "ferrum",   "~> 0.13.0"
+  s.add_runtime_dependency "ferrum",   ">= 0.13", "< 0.16"
 end