Update Mocha

[simison]

Mocha currently: 3.x
Latest: 4.x

This will need some changes to tests files as currently if I just update, it'll throw errors:

/mean/node_modules/mongoose/lib/index.js:391
      throw new mongoose.Error.MissingSchemaError(name);
      ^
MissingSchemaError: Schema hasn't been registered for model "User".
Use mongoose.model(name, schema)