Fixed warning '*' interpreted as argument prefix

- ruby -w lib/multi_json.rb
- lib/multi_json.rb:34: warning: '*' interpreted as argument prefix
- Fixed this warning by wrapping *args by parenthesis
- This warning was found by me here https://travis-ci.org/rails/rails/jobs/7572922#L574
Merge pull request #120 from prathamesh-sonpatki/master

Fixed warning '*' interpreted as argument prefix