fix package.json too be valid json - needed for npm 

Currently npm fails to install the module:
npm ERR! Failed to parse json
npm ERR! Unexpected token }
npm ERR! File: ../mixpanel-api/package.json
npm ERR! Failed to parse package.json data.
npm ERR! package.json must be actual JSON, not just JavaScript.
npm ERR! 
npm ERR! This is not a bug in npm.
npm ERR! Tell the package author to fix their package.json file. JSON.parse