Update server.js (#9)

Fixes DeprecationWarning: `open()` is deprecated in mongoose >= 4.11.0, use `openUri()` instead
See: https://github.com/Automattic/mongoose/issues/5399

server.js

@@ -5,7 +5,7 @@ const bodyParser = require('body-parser')
 const cors = require('cors')
 
 const mongoose = require('mongoose')
-mongoose.connect(process.env.MLAB_URI || 'mongodb://localhost/exercise-track' )
+mongoose.connect(process.env.MLAB_URI || 'mongodb://localhost/exercise-track', { useMongoClient: true } )
 
 app.use(cors())