Resolve redis-rb warning exists => exists?

Resolving this warning message ensuring future compatibility:
```
`Redis#exists(key)` will return an Integer in redis-rb 4.3. `exists?` returns a boolean, you should use it instead. To opt-in to the new behavior now you can set Redis.exists_returns_integer =  true. To disable this message and keep the current (boolean) behaviour of 'exists' you can set `Redis.exists_returns_integer = false`, but this option will be removed in 5.0. (/Users/jonathan/.rvm/gems/ruby-2.7.1/bundler/gems/lit-f3e8ccbf6201/lib/lit/adapters/redis_storage.rb:22:in `[]')
```

Author: usiegj00

lib/lit/adapters/redis_storage.rb

@@ -17,9 +17,9 @@ def initialize
     end
 
     def [](key)
-      if Lit.redis.exists(_prefixed_key_for_array(key))
+      if Lit.redis.exists?(_prefixed_key_for_array(key))
         Lit.redis.lrange(_prefixed_key(key), 0, -1)
-      elsif Lit.redis.exists(_prefixed_key_for_nil(key))
+      elsif Lit.redis.exists?(_prefixed_key_for_nil(key))
         nil
       else
         Lit.redis.get(_prefixed_key(key))
@@ -56,7 +56,7 @@ def keys
     end
 
     def has_key?(key)
-      Lit.redis.exists(_prefixed_key(key))
+      Lit.redis.exists?(_prefixed_key(key))
     end
     alias key? has_key?