This works on Windows/Linux/Mac (Only Desktop Version, does not work with the Web Player or Smartphone app) also. Here's the link: http://textmechanic.com/text-tools/basic-text-tools/sort-text-lines/
1. Just go to the playlist you wanna randomize press Ctrl + A to select all songs or hold Ctrl and select specifically songs by clicking. Then press Ctrl + C to copy.
2. Go to the Web Tool, erase the description text inside, press Ctrl + V to paste your tracks (NOTE: If you wanna backup your current list before the change, go down and click Save As). Once you've pasted the links, click RANDOM as many times as you want. Important note: It doesn't work for local files, cause local files doesn't contain URL.
3. Press Ctrl + A to select all, go to Spotify and you have two choices: 1. Create a new playlist and press Ctrl + V to paste all the new randomize songs or 2. Erase all songs of your current playlist by pressing Ctrl + A then Delete and paste the songs with Ctrl + V.
This tool it's free and you can use it as many times as you want.
I hope this will help you :)