[Security] Bump rack-protection from 1.5.3 to 1.5.5

Bumps [rack-protection](https://github.com/rkh/rack-protection) from 1.5.3 to 1.5.5. **This update includes security fixes.**
- [Release notes](https://github.com/rkh/rack-protection/releases)
- [Commits](sinatra/rack-protection@v1.5.3...v1.5.5)

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

Author: dependabot-support

Gemfile.lock

@@ -45,8 +45,8 @@ GEM
     multi_json (1.12.1)
     multipart-post (2.0.0)
     public_suffix (2.0.5)
-    rack (1.6.8)
-    rack-protection (1.5.3)
+    rack (1.6.9)
+    rack-protection (1.5.5)
       rack
     rack-test (0.6.3)
       rack (>= 1.0)