Explicitly install rack gem (#12972)

Pin rack version to 2.0.8. Rack-2.1.0 and 2.1.1 are known to record error below upon requests to `/sidekiq`:

```
NoMethodError - undefined method `transform_keys' for #<ActionDispatch::Request::Session:0x...>
```

Gemfile

@@ -9,6 +9,7 @@ gem 'puma', '~> 4.3'
 gem 'rails', '~> 5.2.4'
 gem 'sprockets', '~> 3.7.2'
 gem 'thor', '~> 0.20'
+gem 'rack', '2.0.8'
 
 gem 'thwait', '~> 0.1.0'
 gem 'e2mmap', '~> 0.1.0'

Gemfile.lock

@@ -441,7 +441,7 @@ GEM
     pundit (2.1.0)
       activesupport (>= 3.0.0)
     raabro (1.1.6)
-    rack (2.1.1)
+    rack (2.0.8)
     rack-attack (6.2.2)
       rack (>= 1.0, < 3)
     rack-cors (1.1.1)
@@ -749,6 +749,7 @@ DEPENDENCIES
   pry-rails (~> 0.3)
   puma (~> 4.3)
   pundit (~> 2.1)
+  rack (= 2.0.8)
   rack-attack (~> 6.2)
   rack-cors (~> 1.1)
   rails (~> 5.2.4)