Update mocha requirement from ~> 0.14 to ~> 2.4

Updates the requirements on [mocha](https://github.com/freerange/mocha) to permit the latest version.
- [Changelog](https://github.com/freerange/mocha/blob/main/RELEASE.md)
- [Commits](freerange/mocha@v0.14.0...v2.4.0)

---
updated-dependencies:
- dependency-name: mocha
  dependency-type: direct:development
...

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

rotoscope.gemspec

@@ -25,6 +25,6 @@ Gem::Specification.new do |s|
   s.extensions = ["ext/rotoscope/extconf.rb"]
 
   s.add_development_dependency("minitest", "~> 5.0")
-  s.add_development_dependency("mocha", "~> 0.14")
+  s.add_development_dependency("mocha", "~> 2.4")
   s.add_development_dependency("rake-compiler", "~> 1.2")
 end