Update sequel requirement from ~> 5.88.0 to ~> 5.89.0

Updates the requirements on [sequel](https://github.com/jeremyevans/sequel) to permit the latest version.
- [Changelog](https://github.com/jeremyevans/sequel/blob/master/CHANGELOG)
- [Commits](jeremyevans/sequel@5.88.0...5.89.0)

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

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

oxidized.gemspec

@@ -50,5 +50,5 @@ Gem::Specification.new do |s|
 
   # Dependencies on optional libraries, used for unit tests & development
   s.add_development_dependency 'oxidized-web',        '>= 0.14.0'
-  s.add_development_dependency 'sequel',              '~> 5.88.0'
+  s.add_development_dependency 'sequel',              '~> 5.89.0'
 end