Update rack requirement from ~> 1.6.4 to ~> 2.1.2

Updates the requirements on [rack](https://github.com/rack/rack) to permit the latest version.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@1.6.11...2.1.2)

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

Author: dependabot[bot]

Gemfile.lock

@@ -85,7 +85,7 @@ GEM
     pry-nav (0.2.4)
       pry (>= 0.9.10, < 0.11.0)
     public_suffix (4.0.1)
-    rack (1.6.11)
+    rack (2.1.2)
     rack-test (0.8.3)
       rack (>= 1.0, < 3)
     rainbow (3.0.0)
@@ -161,7 +161,7 @@ DEPENDENCIES
   guard-rspec (~> 4.5)
   pry (~> 0.10)
   pry-nav (~> 0.2.4)
-  rack (~> 1.6.4)
+  rack (~> 2.1.2)
   rack-test (~> 0.6)
   rake (~> 10.4)
   rspec (~> 3.1, >= 3.1.0)

auth0.gemspec

@@ -26,7 +26,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'pry-nav', '~> 0.2.4'
   s.add_development_dependency 'rspec', '~> 3.1', '>= 3.1.0'
   s.add_development_dependency 'rack-test', '~> 0.6'
-  s.add_development_dependency 'rack', '~> 1.6.4'
+  s.add_development_dependency 'rack', '~> 2.1.2'
   s.add_development_dependency 'simplecov', '~> 0.9'
   s.add_development_dependency 'faker', '~> 1.4'
   s.add_development_dependency 'yard', '~> 0.9.12'