Bump rack from 2.2.18 to 2.2.20 (#88)

Author: dependabot[bot]

Gemfile.lock

@@ -82,7 +82,7 @@ GEM
       forwardable-extended (~> 2.6)
     public_suffix (4.0.6)
     racc (1.7.3)
-    rack (2.2.18)
+    rack (2.2.20)
     rb-fsevent (0.11.0)
     rb-inotify (0.10.1)
       ffi (~> 1.0)