Update mongo requirement from >= 2.14, < 2.20 to >= 2.14, < 2.21

Updates the requirements on [mongo](https://github.com/mongodb/mongo-ruby-driver) to permit the latest version.
- [Release notes](https://github.com/mongodb/mongo-ruby-driver/releases)
- [Changelog](https://github.com/mongodb/mongo-ruby-driver/blob/master/docs/release-notes.txt)
- [Commits](mongodb/mongo-ruby-driver@v2.14.0...v2.20.0)

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

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

nose.gemspec

@@ -35,7 +35,7 @@ Gem::Specification.new do |s|
   s.add_dependency 'cassandra-driver', '~> 3.2.5', '>= 3.1.0'
 
   # Required for MongoDB backend
-  s.add_dependency 'mongo', '>= 2.14', '< 2.20'
+  s.add_dependency 'mongo', '>= 2.14', '< 2.21'
 
   s.add_development_dependency 'fakefs', '~> 2.5.0'
   s.add_development_dependency 'memory_profiler', '~> 1.0.0'