Fix support for Ruby 2.1

It seems Ruby changed how `nil` behaves between 2.1 and 2.2.

- Ruby 2.1

  ```ruby
  nil.frozen?
  # => false
  ```
- Ruby 2.2

  ```ruby
  nil.frozen?
  # => true
  ```

We anticipate that most values will not be `nil` and most of the Rails
apps are already on Ruby 2.2. Additionally, under heavier loads all but
the first read will already be frozen. So we place the `nil` check as
the 2nd predicate as we usually won't need it.

Author: cupakromer

lib/kracken/controllers/token_authenticatable.rb

@@ -49,8 +49,8 @@ def cache_valid_auth(token, &generate_cache)
       end
 
       def shallow_freeze(val)
-        # nil is always frozen
-        return val if val.frozen?
+        # `nil` is frozen in Ruby 2.2 but not in Ruby 2.1
+        return val if val.frozen? || val.nil?
         val.transform_values!(&:freeze).freeze
       end