Bump rack-protection from 1.5.3 to 1.5.5

Bumps [rack-protection](https://github.com/sinatra/sinatra) from 1.5.3 to 1.5.5.
- [Release notes](https://github.com/sinatra/sinatra/releases)
- [Changelog](https://github.com/sinatra/sinatra/blob/master/CHANGELOG.md)
- [Commits](https://github.com/sinatra/sinatra/commits)

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

Gemfile.lock

@@ -208,8 +208,8 @@ GEM
       coderay (~> 1.1.0)
       method_source (~> 0.8.1)
       slop (~> 3.4)
-    rack (1.6.11)
-    rack-protection (1.5.3)
+    rack (1.6.12)
+    rack-protection (1.5.5)
       rack
     rack-test (0.6.3)
       rack (>= 1.0)