Update sinatra requirement from ~> 2.0 to >= 2, < 4

Updates the requirements on [sinatra](https://github.com/sinatra/sinatra) to permit the latest version.
- [Release notes](https://github.com/sinatra/sinatra/releases)
- [Changelog](https://github.com/sinatra/sinatra/blob/master/CHANGELOG.md)
- [Commits](https://github.com/sinatra/sinatra/compare/v2.2.2...v3.0.4)

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

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -19,7 +19,7 @@ PATH
       rack (~> 2.2)
       rake (~> 13.0)
       redis (~> 4.1)
-      sinatra (~> 2.0)
+      sinatra (>= 2, < 4)
       spicy-proton (~> 2.1)
       statsd-ruby (~> 1.4)
 
@@ -46,7 +46,7 @@ GEM
     method_source (1.0.0)
     mock_redis (0.32.0)
       ruby2_keywords
-    mustermann (2.0.2)
+    mustermann (3.0.0)
       ruby2_keywords (~> 0.0.1)
     net-ldap (0.17.1)
     nio4r (2.5.8)
@@ -105,7 +105,7 @@ GEM
     puma (5.6.4-java)
       nio4r (~> 2.0)
     rack (2.2.4)
-    rack-protection (2.2.2)
+    rack-protection (3.0.4)
       rack
     rack-test (2.0.2)
       rack (>= 1.3)
@@ -146,10 +146,10 @@ GEM
       simplecov_json_formatter (~> 0.1)
     simplecov-html (0.12.3)
     simplecov_json_formatter (0.1.4)
-    sinatra (2.2.2)
-      mustermann (~> 2.0)
-      rack (~> 2.2)
-      rack-protection (= 2.2.2)
+    sinatra (3.0.4)
+      mustermann (~> 3.0)
+      rack (~> 2.2, >= 2.2.4)
+      rack-protection (= 3.0.4)
       tilt (~> 2.0)
     spicy-proton (2.1.14)
       bindata (~> 2.3)