RuboCop: Increase amount of files in cache

Currently, we need to inspect more files that fit into RuboCop's cache:

  $ bundle exec rubocop --list-target-files | wc -l
  25188

Let's adjust RuboCop's configuration accordingly.

.rubocop.yml

@@ -40,7 +40,7 @@ AllCops:
     - 'db/ci_migrate/*.rb'      # since the `db/ci_migrate` is a symlinked to `db/migrate`
   # Use absolute path to avoid orphan directories with changed workspace root.
   CacheRootDirectory: <%= Dir.getwd %>/tmp
-  MaxFilesInCache: 25000
+  MaxFilesInCache: 30000
 
 Cop/AvoidKeywordArgumentsInSidekiqWorkers:
   Enabled: true