use Google OAuth2Strategy to avoid the error that key $t must not start with '$'

" key $t must not start with '$' " error has occured when I signed in with google, because mongodb does not seem to support such value.

Using oauth2 is the good way to avoid this error according to the following discussion. I also confirmed the above error had no longer occured with this fix in my env.

https://github.com/madhums/node-express-mongoose-demo/issues/30

thanks!