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] <[email protected]>

Author: dependabot[bot]

Gemfile.lock

@@ -11,8 +11,8 @@ GEM
     mini_portile2 (2.4.0)
     nokogiri (1.10.10)
       mini_portile2 (~> 2.4.0)
-    rack (1.6.4)
-    rack-protection (1.5.3)
+    rack (1.6.13)
+    rack-protection (1.5.5)
       rack
     sinatra (1.4.6)
       rack (~> 1.4)