Session expiration check

[mariuscoto]

Check so we avoid:

/home/challenge/app/node_modules/mongoose/lib/utils.js:437
        throw err;
              ^
TypeError: Cannot read property 'github' of undefined
    at Promise.gotChallenge (/home/challenge/app/routes/challenge.js:280:43)
    at Promise.<anonymous> (/home/challenge/app/node_modules/mongoose/lib/promise.js:128:8)
    at Promise.emit (events.js:95:17)
    at Promise.emit (/home/challenge/app/node_modules/mongoose/lib/promise.js:66:38)
    at Promise.complete (/home/challenge/app/node_modules/mongoose/lib/promise.js:77:20)
    at Promise.resolve (/home/challenge/app/node_modules/mongoose/lib/promise.js:144:15)
    at Promise.<anonymous> (/home/challenge/app/node_modules/mongoose/lib/promise.js:128:8)
    at Promise.emit (events.js:95:17)
    at Promise.emit (/home/challenge/app/node_modules/mongoose/lib/promise.js:66:38)
    at Promise.complete (/home/challenge/app/node_modules/mongoose/lib/promise.js:77:20)