Add type parameter to LeagueRequest#challenger

LeagueRequest#challenger seemed to be missing the required type parameter from https://developer.riotgames.com/api/methods#!/879/3068 , so I added it and defaulted it to "RANKED_SOLO_5x5".  The other available options at this time are "RANKED_TEAM_3x3" and "RANKED_TEAM_5x5".  

First time submitting a pull request to a well formatted and tested gem, hopefully i'm clicking the right buttons.